I have 2 different resolution monitors (2560x1440 and 1920x1080) and I dislike that my mouse gets stuck at the transition where the smaller display is not aligned with the bigger one. I use cursr to fix that but I can't find an alternative that works on wayland and that's pretty well the only thing stopping me from making the full transition.
I run two different resolution monitors on GNOME and haven't had this issue. My second display also comes and goes from my laptop pretty frequently, and it's been flawless, so I'm not sure where the issue actually lies. If it's in KDE, I'd be looking for a scaling or border setting somewhere to try and figure out the behavior. It may also be that you're extending a control element of some sort (taskbar in GNOME) to the second display, which you'd want to remove.
Changing the scaling either zooms my higher resolution display in too much or zooms the lower resolution display out too much. Maybe the scaling is the only solution though