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.