In general, Spinoza and classical theists do not think God's omnipotence means he can create inconsistent worlds (e.g. he cannot create square circles); omnipotence (for them) means that God can do anything that is logically possible. For Spinoza, he thinks this means that only our current world is logically possible, because if there were other worlds that God didn't actualize, Spinoza thinks this would imply God's lack of omnipotence. For Aquinas, a classical theist, see Article 5 in this section of the Summa [0]. For Leibniz (not a classical theist, but having heavy disagreements with Spinoza), God could create other worlds like Aquinas, but unlike Aquinas, thinks this means that this world is the best of all possible worlds, which is why God actualizes this world as opposed to other possible worlds (Aquinas thinks otherwise - see article 6 of [0]).
For your edit, it's not clear exactly what "nothing is part of both" means in the context of God and his creation. I know it seems like I'm being needlessly difficult and pedantic, but the issue is that God is a being unlike any other. So much so that Aquinas would say that my latter statement is technically false - God is not a being, rather, he is the sheer act of "to be" itself. And so spelling out the relationship between God and beings is tricky---what exactly does it mean for a being to be part of/have being through/exist in/have being from something that isn't strictly speaking a being? Those prepositional phrases, when used in relation to God, clearly have a very different meaning than when we use them to characterize the relation of two beings, e.g. a leaf and a tree. That is all to say: classical theists may not disagree with "creation is a part of God" depending on how they interpret "part of"---if this means "creation has its being in God", then this is something perfectly agreeable to them.
For the moment, let's put any "part of/containment/belonging to" relation between creator and created aside (in part due to the subtleties you note; in part because I'm more interested in the necessity and possibility of creations, and —despite having slept on it— don't understand Aquinas yet).
(in parentheses: sorry for my ad-hoc notation earlier; I've been looking for a survey paper on Algebraic Theology to find out what the conventions are for this field, thus far unsuccessfully)
Article 5: I still don't understand properly, but do see that Aquinas' conclusion is that our creation is not the only possible one (and is therefore contingent?) See note about maximum vs. maximal below.
Article 6: IIUC: things consist of their essence and their accidents. If two things share the same essence, we say one is greater than the other if its accidents be greater. It is possible to "max out" the accidents, however, and in that case to create something "greater" one must create something with an essence which is better than the original essence (as 5 is greater than 4; natural numbers having no accidents).
[I fail to find Aquinas' reply to objection 4 convincing, or even necessarily relevant? As far as I do follow it, it does imply that "greater" is only a partial order, as Mary and created happiness (whatever that may be) are both coatoms in this order: anything which is at least as great as Mary must be either Mary or God, and similarly for created happiness] see https://en.wikipedia.org/wiki/Atom_(order_theory)#Coatoms
As the reply to object 3 shows, this creation may be the greatest possible creation of its essence, but even if God is limited to creating only the "best" creations, he can always create a universe whose essence is greater than that of ours. (here I am imagining that the essence of a universe might be the set of the possible essences within that universe, and the accidents of the universe would be the set of accidents of incidents of those essences, and the set of all the relations between them) So for instance, our universe is the best possible universe with only 7 colours, but God could create a universe with an 8th colour ("octarine" without loss of generality), and then he would have to create a "best possible" universe under those constraints (todo: the maximum universe, or merely a maximal universe?), leaving open the choice of creating universes yet greater.
For example (taking a small model because we lack infinite wisdom), we could say that possible universes are grammatical english sentences, the essence of a universe being the words contained in it, and the accidents being the position of those words within the sentence. Furthermore, sentence A is greater than sentence B if it can be formed by adding words to B, without rearranging B. Examples: "See Spot run" is greater than "See Spot"; "The man upon the stair wasn't there" is greater than "The man wasn't there"; "See Spot run" and "Dick and Jane" are unrelated: neither is greater than the other. Essence A is greater than essence B if all the potential words of B are also potential words of A, so "See Spot run quickly" is greater than "Quickly, see!" even though there is a rearrangement, because its essence is greater.
From this model we recover Aquinas. Given many essences, it is possible to create the greatest sentence using only those words; {"Run","See","Spot"} has the sole possibility of "See Spot run"; {"fast","he","is","very"} has no greatest sentence, for one can always add another "very"; and {"Dick","Jane","kisses"} has two greatest sentences, for "Dick kisses Jane" and "Jane kisses Dick" are incomparable, however, there are greater creations, for {"Dick","Jane","kisses","passionately"} is a greater essence (with 6 maximal creations).
I think what you've said is generally correct in that: yes, Aquinas thinks that making a thing better is possible insofar as its accidents can be made better, but not its essence since changing its essence would make that thing cease to be that thing. Your sentence analogy is apt. Aquinas does indeed think creation is contingent (though someone like Leibniz does too---it's just that Leibniz thinks God is obligated to actualize the best possible worlds as opposed to other possible worlds, but such worlds are still possible, and thus contingent.) The idea that creation is necessary is somewhat fringe (among theists and atheists alike). The idea that all things that are possible are necessary, modal collapse, is generally seen to be so untenable that if a consequence of an argument is modal collapse, it's almost seen as a reductio ad absurdum. This is the case for, for example, Gödel's ontological proof of God.
Maybe one relevant item here is that Aquinas thinks God is the most perfect, and that God cannot make creation to be as good as him---so while it might be true that God can make something that is better than a particular thing (by making something with a different essence), he cannot make something infinitely perfect (the only such thing is himself).
To be sure - God and creation being indistinguishable isn't something that Spinoza would agree with either. Two relevant items here.
One is that God has an infinite number of attributes, only two of which are extension and thought (the only ones we can know). All physical things are modes of the attribute of extension, and all "mental" things are modes of the attribute of thought. So if creation just means the set of all physical things (or even "mental" things), these are just the set of all modes of extension, an attribute of God, (along with the set of all modes of thought, another attribute of God, if you include mental things). But these are but extension and thought are but two of the infinitely many attributes of God.
Two is that Spinoza draws a distinction between the set of all of the modes of the attributes of God (Natura naturata) and God himself (Natura naturans). (See the Note of Proposition 29 of Part 1 of Ethics.)
So if by pantheism we mean that God is nothing but the set of all things, Spinoza does not seem to be a pantheist.
That isn't to say that Spinoza is just a perfectly orthodox theologian, of course, but just that reducing the radical nature of his theology to the word "pantheism" (construed as creation and God being the same) appears to be an oversimplification.
(Also, minor note, in the last comment I said Leibniz thinks God actualizes the best possible worlds, that's a typo, I meant the best possible world, singular.)
Rough going so far, as Spinoza is both precise and uses words in the senses they had in Scholastic times.
I think I understand how Spinoza is not a pantheist, though: God determines Creation, in that the Creation is the maximal object that could be created from God, and the Creation determines God, in that God is the minimal object that could create the Creation, so they're two very distinct things, but in a 1:1 relationship.
(examples: intension/extension in logic, cones/gamut in vision, grammar/language in informatics)
I'm not sure how easy the distinction between his viewpoint and pantheism would have been to explain to someone with a XVII background, but Aquinas defo would've been unhappy with the "the deity can create any universe he wants, as long as it's black" part.
(hmm. if our universe only uses up two attributes, 'thought' for all the abstracts and 'extent' for all the concretes, are there any other universes lurking in the remaining infinity of attributes? this also opens the question of whether we're dealing with a countable infinity of attributes, or a higher cardinal...)
>Creation is the maximal object that could be created from God
Well, just the only one, but I suppose the maximal element of a set with one element is just that element, so yeah.
>Creation determines God, in that God is the minimal object that could create the Creation
Well, the only thing that could create Creation, but I suppose the minimal element of a set with one element is just that element, so yeah.
>Aquinas defo would've been unhappy with the "the deity can create any universe he wants, as long as it's black" part.
Hmm, I don't know what you mean by this. Spinoza isn't saying this, Spinoza is saying that God must have created only this universe, and everything that God does is necessary---there's nothing else he can do.
>if our universe only uses up two attributes
OK, so it's not that our universe is only uses two attributes, it's that we can only perceive things with respect to two attributes. There's only one possible universe, which is this one, and it's necessarily this one.
Yes, I was purposely playing with the trivial definitions of maximal and minimal (max{x} = min{x} = x) to draw out an analogy with the modern concept of Galois Connections.
> there's nothing else he can do.
Right, and I thought Aquinas' take was that he must be capable of doing otherwise, but doesn't.
> we can only perceive things with respect to two attributes
Aha, so this universe uses all (are they countable?) attributes, but we imperfect finite creatures only perceive thought and extension; angels might perceive three, or a dozen, or all the attributes? (which would make sense: Samuel Johnson can both think of a rock and perceive its extension of mass in space, but the rock, while it might "perceive" the extension of Johnson's foot in space does not think of Johnson. So a creature beyond us as we are beyond the rock might have metathought? "be not afraid" indeed!)
That's another reason to introduce the maximal and minimal: those def'ns would still go through even with an Aquinas-style god.
(and they yield an irreligious 1=3 trinitarian argument: if C takes a god to its maximal creation, and G takes a creation to its minimal god, then CG and GC are not necessarily the identity, but we do necessarily have C = CGC and G = GCG)
Definitely, Neoplatonism was a big influence on Abrahamic philosophical thought. Though the real locus classicus for God is Aristotle's Metaphysics, specifically his description of the unmoved mover. There are some not-so-minor differences between Aristotle's theology and Abrahamic theology, but God insofar as he is the ground of all being is a concept that most clearly originated in Aristotle's work.
oops, I was reading the "contingent things must be dependent upon necessary things" as implying modality, but you're right that if it's there it's only implicit.
right, there is something modal going on here, but in terms of the formal mathematical tool of modal logic, Spinoza isn't using them, though I think you understood that from the start.
For your edit, it's not clear exactly what "nothing is part of both" means in the context of God and his creation. I know it seems like I'm being needlessly difficult and pedantic, but the issue is that God is a being unlike any other. So much so that Aquinas would say that my latter statement is technically false - God is not a being, rather, he is the sheer act of "to be" itself. And so spelling out the relationship between God and beings is tricky---what exactly does it mean for a being to be part of/have being through/exist in/have being from something that isn't strictly speaking a being? Those prepositional phrases, when used in relation to God, clearly have a very different meaning than when we use them to characterize the relation of two beings, e.g. a leaf and a tree. That is all to say: classical theists may not disagree with "creation is a part of God" depending on how they interpret "part of"---if this means "creation has its being in God", then this is something perfectly agreeable to them.
[0]: https://www.newadvent.org/summa/1025.htm