Anything that works with Gadgetbridge is a good choice.Or maybe pinetime or bangle.js those look pretty cool too.
I've been using using firefox and more recently fennec and mull on android for years now and I've never had a problem with it. Why do you think it sucks? Just the fact that you can install extensions on firefox (or fennec or mull) makes it by far the best browser in my opinion. I do agree that firefox by default has some privacy issues but there's no way it's worse than chrome which almost everyone uses, and as you said you can just use a fork like mull for best privacy.
There isn't a best distro that supports surface hardware. You have a to install a distro of your choosing for which there is a guide on the linux-surface github and then follow the guide.
There. Is. Not. A. Single. Browser. That. Values. Your. Privacy.
Librewolf, Hardened Firefox, ungoogled-chromium
I really don't understand why people use and recommend brave. They've have had plenty of shady practices in the past and bundle crypto crap by default. If you want to block ads just install ublock origin on (preferably) any privacy respecting browser like firefox (or better yet librewolf), ungoogled-chromium etc
The biggest annoyance to me is how small the border around windows is. On Windows, I can grab anywhere around the edge of a window and resize it, including in both directions from the corners. In Linux, I need an electron microscope to find the edges, and the hand of god to find a corner.
This is a problem with the DE that you use and not linux in general. Gnome and KDE for example don't have this issue. I'm guessing you're using xfce?, since I had the same issue with it.
Afaik most distros don't support surface computers by default. You should check out https://github.com/linux-surface/linux-surface