Excluding Chrome, Firefox and Safari means that you are now relying on some random developer to understand security and privacy and as a software developer for over 40 years I can tell you that this is a fools errand.

Don't get me wrong, the big three absolutely have privacy issues, but they can be mitigated in many different ways without compromising on security.

For example, you can force DNS requests to one of your choosing, you can run them in incognito mode, refuse cookies, run them inside user accounts without personal information, etc.

I tend to run individual instances of a browser in incognito mode and am very conscious of which tabs are open in which instance, so websites cannot steal information from other tabs.

The built in browser on GrapheneOS, Vanadium, is great.

If they're using Bromite, that's a problem because Bromite ended development like a year ago, and they should at least switch to Cromite

Edit: A problem with all the plugins is that it creates a unique fingerprint, which Cromite and Vanadium prevent by not doing plugins/extensions and instead using built-in ad blocking (which can/should be supplemented with a decent VPN and/or DNS level ad blocking)

Keep in mind that when you use an offshoot browser that doesn't have a lot of users, this also makes you easier to identify. How many people use Waterfox? Sure, I have it installed and sometimes use it. You use it. But the small number of users makes us more unique, not less. The plugins that you use further enable fingerprinting, as do hardware specs such as screen size / resolution.

I am not an expert in this area. I think there's a lot going on that hasn't been considered.

I'm using Mull but am going to switch to Fennec with arkenfox hardening and ublock enabled in the future.

IMO don't use too many privacy plugins like ublock, privacy badger and jshelter. Ublock can also block individual scripts in expert mode and therefore reduce your fingerprint to a minimum. Additionally more extensions also mean more consumed ressources.

I would also reccommend a secure dns filter like nextdns or the one from mullwad. I use Rethink dns for the configuration. https://www.f-droid.org/en/packages/com.celzero.bravedns/

Here is a site from Germany that handles many privacy topics and has a section for dns servers: https://www.kuketz-blog.de/empfehlungsecke/#dns

someone compared Chromium and Firefox without any real conclusion

Are we talking about mobile or desktop? Because, as of now, Chromium is still winning on mobile