The thing is, flags in Chrome exist only until Google decides it shouldn't be an option anymore. Then they take the choice away from you. I disabled the material "design" styled themes for as long as I could, until the flag disappeared.

That's not really different for Firefox, there's just a different organization making that decision.

Chrome flags though die out so fast there's virtually no point in exposing them at all, it's disappointing and frustrating almost every release.

Firefox has been killing off features year after year I dont see why you trust them to be better in that regard.

It’s not remotely as complete as about:config

chrome://about/

That's just about:about, not an about:config replacement.