> Is this really not a mostly “solved” problem using assistive browser (or OS-level) ... > ... performatively sacrifice aesthetics ...

One doesn't need to sacrifice aesthetics to make the design legible to an average user.

> At what point is it enough?

It depends on who your audience is. The more people you want to reach, the more you need to cater to everyone's needs. There are fundamental anti-patterns to avoid like writing with a dark font on a dark background.