Been using Vivaldi [0] on and off for the last few years. Now trying to run it full-time the last couple weeks (thanks to Microsoft's questionable Edge "additions" and avoiding Chrome for any non-Google sites).
Still has a couple quirks I'm getting used to (e.g. no touchpad gestures for history navigation), but the customization is great. It is partially closed-source, but they are open about what they do [1].
[0]: https://vivaldi.com
[1]: https://vivaldi.com/blog/vivaldi-browser-open-source/