Hello I had a similar problem with a laptop and after digging a bit it came out to be a issue with the latest ddcutil (2.2.3-1 ā 2.2.5-1) that makes kde plasma freeze after login.
You can try to roll back to the previous version, or restore from backup and update everything except ddcutil.
sudo pacman -Syu --ignore=ddcutil
PS - Looks like an official workaround of disabling the ddcutil was also posted in arch wiki here: https://wiki.archlinux.org/title/KDE#Disable_ddcutil
Hope it helps