Just because Brave is based on Chromium doesn’t mean it’s dependent on it. Chromium is open-source and Brave can modify it. The only thing Google can have an impact on is updates, but if you think about it for two minutes, there’s not much Google can do about that. Manifest V4? As I said, Brave’s blocker is integrated into Brave, so it will continue to work. Google can’t magically integrate telemetry into Brave either.
Don’t forget that Chromium remains open source. It’s not really a monopoly, since several browsers use their own version of Chromium.
So what can it do against Brave Opera and all the others? Not much, really, apart from the security updates the browser needs. You say Google could make big changes, but what changes?