I actually tried KDE Plasma on 20.04 LTS just a few days ago, and it has HiDPI bugs. When moving the mouse pointer over the desktop the pointer is drawn too small rather than 200% scale. I actually like the KDE taskbar better than Gnome's default. (I ended up installing the Gnome dash-to-dock extension to get a KDE-like taskbar.) I've tried Gnome, KDE, Xfce, and old-school window managers (mostly FVWM2) and Gnome has the fewest overall issues (particularly for HiDPI) but it still has a large number of bugs, especially when installing user themes, which I learned to just avoid for the sake of my sanity. (I actually tried KDE because I wanted to avoid installing dash-to-dock on Gnome, because of how often I've seen extension bugs on Gnome.)

Wayland still isn't 100% there for nVidia yet. "Well don't use nVidia then!" Well, for many machine learning workflows and other use cases non-nVidia isn't 100% there yet. But yeah, Linux users should avoid nVidia to the maximum extent possible.

Your Firefox tip sounds good, thank you.