I don't think Vivaldi is open source. But if Brave's features are so bad that you're willing to switch to an closed source browser, why not switch to Firefox?

So when I'm using OpenBSD, I use Firefox exclusively. On my Mac, I use Safari 99% of the time, but I do have Firefox installed and use it for some things, but when I need a Chromium-based browser, I use Vivaldi, instead of Chrome or Brave.

EDIT: It looks some of Vivaldi's source is available FWIW, https://vivaldi.com/source/

The UI is closed source.

https://vivaldi.com/blog/vivaldi-browser-open-source/

Would you happen to also know what the Vivaldi source code contains then?

Vivaldi is even worse for privacy (and it's not even fully open source). You're better off just using Chrome.

https://privacytests.org/

These tests just show the default settings. If you enable the ad blocker and disable third party cookies it would score a lot better.

Then if you add the same to Chrome, it'll be even better.