Many forks of Chromium, yes. Most if the rest of them pass the “open source” test but would struggle on the “browser” side.

Zen Browser, Floorp, Mullvad Browser, Pale Moon, Epiphany aren't "browsers"?

Don’t forget the most well-funded alternative, Edge!

We're talking about open source browsers.

Mullvad Browser is basically Tor Browser without the onion network which is itself based off Firefox

Who pays for the development? Is the linux model viable for a browser?

Open source browsers are either bad and non-competitive, or they're Firefox and still get criticism for being in the pocket of Google.

Brave or Vivaldi aren't bad. They get money from partnerships, search deals and donations.

They also avoid actually paying for browser development. Could Brave or Vivaldi fund Chromium itself?

I don't know, but they already contribute patches to Chromium.