as an experienced graphic designer (just getting into UX) who is also autistic, i can appreciate and agree with most of this.

i think these days, identity first is much more common but you're right, it is usually preference and best to ask (if you can). if not, assuming identity first is probably a good bet. person first is typically better for curable diseases (like 'personw with cancer' for example).

i can't speak for every disability, but among autistics, i think simplicity is quite important and wildly underrated among a ton of designers i see. keep the random pop up at bay, don't have surprising elements, flashing elements, moving elements, etc. i also think though that if more people designed for the more sensitive of the population (such as autistics), the rest of the population would benefit and appreciate it as well.