Honest question out of curiosity-
Are there possibly AI-based solutions possible that can add assistance at a more generic level that doesn't require software that has "deep" knowledge of the window architecture (and the actual text in it etc.)? My understanding is that this is how tech like VoiceOver works- it knows the actual window definition and all the elements of it at a programmatic level and can take advantage of that.
I'm not asking if they're already available (although that would be a nice option), but for projects like this that wanted the speed of rendering on the GPU (at the possible cost of, as you said, VoiceOver seeing an "empty window"), it would be at least a fallback position.
(So does this mean that ALL content that renders through the GPU, such as games, are inaccessible to you? If so, I'm sorry...)
Not sure if this might help you but I have an Apple shortcut defined on my iPhone called "GPT Explains" that is activated by a double-tap on the back of the phone (which you can assign, as you probably know, in Accessibility settings)- it takes a screenshot, ships it off to OpenAI and returns with a description of what it's seeing, any to-English translation of non-English text, and any counterarguments to any claims made in a meme, etc. (yeah, the prompt for this is kinda wicked, lol). If this is helpful to you, I can give you a link after I remove my OpenAI key (you'd have to provide your own).
EDIT: I made a copy of it without the API key: https://www.icloud.com/shortcuts/0d063c6810d74a35a017e5a5f69...