I hear you on the concern about the browser engine monoculture. It's a concern of mine as well.
I guess what I should say is - _if_ you're going to use a chromium-based browser, you should probably use Brave.
I'm currently looking into Mull browser as it seems to be to Firefox what Brave is to Chrome.
Might actually give it a second chance now that Google is clearly hamstringing non-chromium browsers on Youtube. Both Firefox and Safari on macOS have weird loading issues - and I pay for Premium so it can't be adblock related.
So going by a letter of definition Chrome Brave is a fork, but by spirit of a definition it is not. Example - this topic right here. If it was a fork they would have forked the code and maintained Manifest V2 instead of V3, autonomously, like software forks do. But since it is not a real fork (in spirit of the word), they can't and don't want to do it, and simply accept every change the real code owner does, Google in this case.