This + mixed refresh rate are the key selling points of Wayland.
Xorg does per-monitor DPI and per-monitor refresh rate. Debian probably never shipped a version that does, but it works fine on Gentoo Linux.
I've tested per-monitor DPI before, and [0] mentions one way to do it. I tested per-monitor refresh just now. Using the xrandr CLI to set the refresh rate to 24.0 on my primary monitor and 60.0 on my secondary results in "cinematic" visuals on the primary monitor and "soap opera" visuals on the secondary.
I'm currently using Windowmaker, but I see no reason why this wouldn't work with KDE.
I agree it used to be fiddly at best but in recent years I had found it to be pretty easy in X11.
I haven't had complaints there for Wayland but I will say that it breaking other things has been annoying.