We could change the Brave user-agent to resemble the Tor Browser, but Brave and the Tor Browser behave differently enough that that wouldn’t really improve anonymity — with some effort, Brave is even distinguishable from Chrome, on which it is based, so a completely different browser foundation like Firefox is going to be even easier to tell apart.
What Brave can do is make Brave users hard to tell apart from each other. If you want to blend in with Tor Browser users, the best way is to use the Tor Browser.