Perhaps have a look at Pop!OS - still not Wayland but all the good stuff of Ubuntu and none of the snaps.

X11 has been able to do mixed resolution from the beginning.

http://wok.oblomov.eu/tecnologia/mixed-dpi-x11/

You're right I did face some issues too with this when one of the monitors was different but the solution was rather simple. Just had to add a Modeline in some file called 10-monitors.conf(1) and then it worked perfectly until I replaced the 4th monitor with the same configuration, then no such hack was required.

You're right it didn't work out of the box but it was very simple thing even though I'm relatively new to linux myself (relatively speaking)

(1) https://gist.github.com/r0xsh/4d1c7219ee63e4cee0c7fe1077559b...

It's possible I am misunderstanding something, but you can do mixed monitor resolutions on Linux Mint Cinnamon. I usually have two monitors hooked up to my desktop, a 1440p resolution monitor and a 1080p monitor. The 1440p monitor is running at 144hz with the 1080p monitor running at 60.

It is a bummer that it's not on Wayland, and seemingly they have no current plans on migrating.

You may want to give ARandR a shot: https://christian.amsuess.com/tools/arandr/