On the arch wiki for SSDM I found this:
Changes to your display configuration made in a Plasma Wayland session (e.g. monitor layout, resolution, etc) will not persist to SDDM. To make them persist open Plasma's System Settings and navigate to Startup and Shutdown> Login Screen (SDDM) and click "Apply Plasma Settings...". You will need to have permission to perform this action.
You should give that a try
I've just been logging in upside down for a couple years. My monitor's vesa Mount is like 3 inches from the top for some reason so having it upside down is the only way I can get a reasonable ergonomic height
Which display manager are you using?
That's what timeshift and btrfs is for! Really though it takes like ten seconds to roll back and each snapshot only takes like 40mb. There's a pacman hook to take a snapshot before updating.
AUR is just incredibly convenient for me. I don't have to think about it, I don't have to track anything down.
I'm going to go against the flow here and say BTRFS. It's stable enough to the point of being a non consideration. You get full backups using a negligible amount of storage. Even using it on Windows is easier than using ext4 with the winbtrfs driver.
If you have to read PDFs regularly having a vertical monitor is a lifesaver.