For what it's worth, egui does have an accessibility layer. I haven't gone to any particular effort to wire it up yet (besides what you get by default): https://github.com/emilk/egui/issues/167
So what are you doing that couldn't be performant in the DOM? Because any kind of fast updates that you're doing with a lot of triangle that are too fast to render into an XML tree are also going to be happening too fast to describe with a screen reader.
The DOM is asking you to be able to describe your interface -- performantly -- in entirely pure text. If you can't do that, then there's no way you can be fully accessible, because the screen reader needs to be able to describe your interface -- performantly -- in entirely pure text.
If you are updating thousands of rectangles in your UI at 30/60fps, it is impossible for you to fully convey the same amount of information that is changing visually to a screen reader.
Either your interface can be described using pure text, or it can't. Either I could use your interface by closing my eyes and having someone describe it to me, or I can't.