In the case of Brave and Vivaldi, they add their own undesirable parts (Brave adds crypto bullshit and Vivaldi is closed-source, so $DEITY knows what they’re adding).

Librewolf is open and doesn’t contribute to the Chromium monoculture; so it’s the best option

they are still based on Chrome or Firefox doesn’t really matter, since they remove the undesirable parts of them.

Actually it does matter, since Google are killing off the ability for ad blockers to work with a new manifest specification in Chromium.