> Consciousness is the sheer fact of subjectivity (i.e., that it is "like something" to be you).
That fact doesn't necessarily entail that it cannot be captured objectively. So your conjecture is that our apparent subjectivity is irreducible, that we must simply accept subjectivity into our ontology, and that's a stretch.
The problem, as I see it, is this: you can be sure consciousness exists (try and doubt it, and you will only discover the conscious experience of doubt), but you cannot be sure that anything else does. I cannot see why it's sensible to reduce the one certain thing to any number of uncertain things.
In a dream, you may similarly find that certain "objective" phenomena correlate near-perfectly with consciousness. Does this constitute consciousness being "captured objectively" by them? If not, why is the situation any different here, where we cannot be certain we're not in a simulation? (Notice that my argument does not depend on the likelihood of the simulation hypothesis, but just the possibility.)
> The problem, as I see it, is this: you can be sure consciousness exists (try and doubt it, and you will only discover the conscious experience of doubt), but you cannot be sure that anything else does.
This is a pretty shallow interpretation of your perceptions I think. A Cartesian cogito argument doesn't prove that you exist because it assumes the conclusion (and so does not prove subjectivity exists).
A non-fallacious cogito argument is, "this is a thought, therefore thoughts exist". Where does the subjectivity arise in this proof?
At best, you can prove "this is a thought asserting the existence of subjectivity, therefore thoughts asserting the existence of subjectivity exist". This clearly does not entail that this belief is actually true.
So in conclusion, I dispute your certainty that consciousness exists. Many things seem certain at first blush, but turn out to be false upon closer inspection.
And if you want something more concrete to chew on, I recommend reading [1], which is a neuroscientific theory that attempts to account for our apparent experience of subjectivity.
I think for the most part we're remarkably adept at not noticing what's going on with our own experience. If you stop to notice how you recognize that you had the thought "this is a thought asserting...", you will discover what it is I mean by the word subjectivity, and that even your refutation of it becomes known to you via it.
Instead, I think we're remarkably adept at thinking we know what's going on with our experience. Our brains are constantly filling in the holes of perception and memory with reasonable falsehoods that are readily accepted. This is a documented fact with overwhelming evidence, so on that basis alone I would be skeptical that you know you are conscious and what that means.
Ultimately, either your meaning of "subjectivity" can be captured by an objective description, in which case you and I agree on the nature of subjectivity, or it cannot be so captured and you agree with the meaning of subjectivity in the philosophy of mind.
Finally, the mechanics of metacognition merely require an additional level of indirection, no magical secret sauce like subjectivity is required. We can even perform metalogical reasoning of a similar sort within basic formal systems like arithmetic via embeddings. I don't know why you think a whole new category of cognition is necessary.
Your position seems to be that, because any sort of explanation of consciousness may be uncertain, we should not even make the attempt. That is not an argument for any position, it is an attempt to avoid addressing the question.
I trust that we will one day find precise physical correlates of, and conditions for (i.e., "causes of"), consciousness. But the burning question I have (roughly: "what IS this thing?!) will not have even been partially satisfied, any more than if the same happened in a dream.
I think guys like the Buddha were right: for that, I have to sit down for a good long while and allow consciousness to become more fully aware of itself. And if it cannot be communicated to others, that's perhaps just how it goes.
That fact doesn't necessarily entail that it cannot be captured objectively. So your conjecture is that our apparent subjectivity is irreducible, that we must simply accept subjectivity into our ontology, and that's a stretch.