I wonder how the Helium [0] project solves this, since they also work off of an opinionated-ish Chromium base.

[0] https://helium.computer

I only glanced at Helium's site, but I suspect from what I saw that they pick a status (enabled/disabled) for all those optional features, meaning they just have to test one (or maybe two or four) combinations of `--enable-foo` / `--enable-bar` flags. Whereas Gentoo tried to allow users to set whatever combination they wanted, and that wasn't working so well when upstream wasn't helping.