> Relatedly, “an autistic” is generally considered dated / mildly offensive / just incorrect.

Is it? I've heard those sentiments about "a person with autism" (and more generally, "autism" as something you have rather than "autistic" as something you are), but not about the term "autistic" as a noun in general. I use "autistic" as a noun a lot, because I don't like to assume that everyone identifies as a person. (Even though doing so is normalized enough that not doing it looks more out of place.)

> There does exist the noun “autist” which I do hear occasionally, but not from autistic people as far as I’m aware, so would probably avoid as well.

In my experience "autist" doesn't necessarily have to do with autism itself, but more specifically the element of "weirdness"/"cringe" that others can have at it; i.e. it's usually used in a self-deprecating way to refer to some sort of weird or deranged behavior (and isn't at all a neutral way to refer to others). Though, most of my exposure to the term is from 4chan/2b2t; in those spaces autism in general is often used as a synonym for mental illness.