https://wiki.archlinux.org/index.php/HiDPI has lots of information.

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

The silly 0.9999x0.9999 scaling on the first screen is to work around a mouse flickering bug in the modesetting driver: https://gitlab.freedesktop.org/xorg/xserver/issues/70

Is that about weird multistream displayport connections? Two times QHD is definitely not vanilla and was unsupported on windows until a while ago as well.