Day 11 after my M1 mini upgrade from Big S(o)ur to Monterey. Rock solid. Not a single reboot, whereas I barely made it to Day 4 before. All the randomly recurring bugs are gone.
I had planned to rebuilt the M1 after the upgrade. Obviously, not needed.
these big upgrades normally replace all core OS files, so anything that was damaged before will have been replaced. My M1 mini has been rock solid, but have see a few kernal panics over the year, which I think have all been software and not OS related.
New Update:
About 10 days ago, Monterey started to act up again. Very bad this time, lots of hiservices-xpcservice hanging messages, system became very unstable, Finder issue recurring in full force, etc. Reboots every 24 hours or even more frequently were needed.
Eventually, I found out that Clean My Mac had some services/processes running that grabbed >6GB memory and failing right before the hiservices-xpcservice messages appeared. I uninstalled Clean My Mac a week ago and have not had a single issue since.