I long ago decided that the Compose key is the best solution to this problem. In many cases, the Compose sequence is intuitive, e.g., Compose+l+- gives you £, Compose+t+m gives you ™, Compose+*+l gives you λ, and Compose+e+' gives you é. Compose is easy on Linux/BSD (some variation of setxkbmap will generally let you remap a key as Compose), and there's a WinCompose for Windows which behave essentially identically. There's a sort-of-solution for MacOS using Karabiner, but I've forgotten the details.
The thing about Compose is that it fits in with touch-typing, whereas choosing the character from a menu means you have to stop, look at the menu, and select the desired character, before going back into typing mode. Perhaps that's good for people who almost never use non-ASCII, but that would definitely slow me down.
You don't need to look or wait, you can blindly pick the familiar key of your remember, say, that u is always umlaut to the previous key, same as in Compose
The big issue with pure Compose solution is that it fails for rarely used or for many symbols - hard to remember all those combos. You must have a visual reminder
The ideal combo is being able to delay showing GUI so that for combos you remember well nothing will popup at all, so it will be the same as "pure Compose", but don't think that's implemented anywhere
So the only remaining difference is pure type combos vs hold trigger, but you don't need to pick: you can have both
3. hold "a", wait till menu pops up, release "a" freely (waste of an ergonomic effort to require to hold it) and then do whatever you like more
- press a symbol label to insert it
- use cursor keys, including those bound to your right thumb+ home row to select and enter (or right thumb + another near home key) to insert
- use your mouse to pick a symbol
- press a key to search by name and have an option to add it to "a" list
- press a key to see recent/favorites
(and the popup is not a 1-dimensional list, but can show you your keyboard layout so it's easier to find the key that corresponds to a symbol)
I type regularly in English, Portuguese and Spanish. 2 out of 3 of those languages use a lot of diacritics and other character modifications (ç, ñ, ...).
I also use the repeat key and wouldn't want to not have that available.
Compose keys can work but they add one extra keystroke for each modified character. I prefer to use keymaps with dead keys for that, so to type á I press ', then a; to type ñ, it's ~, then n; and so on and so forth.
I then have a shortcut to change keymaps for when I'm typing English or writing code vs. when I'm typing Portuguese or Spanish.
Long-pressing is terrible. It introduces a delay which massive slows down my typing and overrides the key repeat.
Mac actually does this already - hold down a letter key and you can select a variation using the number keys. I guess he was on to something back in 2011!
Alt codes are great for rarely used characters. For common accents for instance, moving to something that works within two to three keystrokes or a swipe is a game changer.
macos does it pretty well with Option＋key combinations to insert an acute/grave symbol for instance. Otherwise I think most people add shortcuts/custom key mappings to deal with it when their keyboard doesn't have the keys.
These are awful, why would anyone have to remember numeric garbage instead of visually picking a symbol from a list/table or search by name/pick from favorites /recently used?
MacBooks now apparently default to character variant selection rather than repeating when you hold down a key. This is a nightmare when navigating in vim.
The thing about Compose is that it fits in with touch-typing, whereas choosing the character from a menu means you have to stop, look at the menu, and select the desired character, before going back into typing mode. Perhaps that's good for people who almost never use non-ASCII, but that would definitely slow me down.