Sounds like you want to go back to tweaking an Xorg config file 🙂 A Monitor section has an option for a DisplaySize (in mm) that may be used (overriden by dpi from DDC file so may have to be disabled) but since the dimensions seem to be detected fine already I’d fiddle with xrand and if you’re happy with the results put that in ~/.xprofile or whatever. Here you can set anything without restrictions from the GUI. Example:
xrandr --output DisplayPort-0 --scale 0.8x0.8
Sadly this doesn’t seem to work with Wayland (I tried it even with XWayland for you). There was a project to rewrite xrandr but looks like it never flew. Latest solution to this seems to be wlr-randr that has according to source a --scale parameter but this requires wlr-output-management-unstable-v1 and while this may be in wlroots by now it’s not shipped (for me!) yet. Your mileage may vary.