KDE honestly seems the best for this, although on the latest version of GNOME has fractional scaling available behind a flag (although also buggy and very slow).
https://blog.system76.com/post/168340008923/hidpi-is-release...
Testify :(
I'm still struggling to get my (IMHO, extremely vanilla) setup working (Ubuntu on Thinkpad T430 + 2 external QHD monitors), and I'd appreciate any pointers to any resources that can help me resolve this.
I have a Thinkpad X1C and an external monitor, both with QHD resolution. What I ended up with is using xrandr scaling to increase the resolution of the external monitor (probably should have gotten a 4K instead...), and tweaking font sizes and Firefox's UI scaling until it looks okay on both screens. For QT apps I also had to set QT_AUTO_SCREEN_SCALE_FACTOR=0. Some proprietary apps are still too small though, so I tend not to use them ;-)
The xrandr scaling is applied through a script and not persistent, so I mapped it to a keyboard shortcut I can press when I reconnect the monitor.
My script looks like this:
xrandr \
--output eDP-1 --pos 0x0 --scale 0.9999x0.9999 \
--output DP-2 --pos 2560x0 --scale 1.25x1.25
Works fine for me on i3 (X11).
That assumes non-Nvidia hardware. If you have one of those you are on a potentially very different timeline, that I do not dare to predict.