The way Google handles the Chromium source (one big Git monorepo that's undergoing near-constant refactoring) means anything that doesn't want to hard fork is limited to bolting on a new UI and features around the core rendering engine -- but anything inside that core rendering engine won't be long-lasting.

So Brave, Arc, Dia, Edge, et al., all add their own sauce on top of Chromium, but none of them can make changes like back-porting Manifest V2 back into their Chromium.