Yeah, I've been running Plasma as a daily driver (and without a fallback OS) for around 8 years now without this kind of trouble. I routinely run for months between reboots, and when a reboot happens, it's usually because I did a dumb.

I helped someone else earlier today with an unstable Linux laptop, it turned out there was an amd/gpu/drm issue that was crashing Wayland in the background.

My first bet would be that you've got something similar going on -- a hardware or device driver problem. You've crawled through journalctl and the like, I assume?