My recommendation is the opposite of @Mirdarthos’s suggestion - it is to remove Plasma Frameworks 5 leftovers. This is what worked for me when I had the same issue of a custom SDDM theme not allowing me to log in when I updated from Plasma 5 to Plasma 6:
Log in to a TTY session (usually Ctrl-Alt-F3) as your normal user (not root).
Then run the command sudo pacman -Ru kf5 to remove any remaining vestiges of Plasma 5 frameworks.
Then run startx which should log you into a Plasma 6 X11 session.
Then open KDE’s System Settings => Colors & Themes => Login Screen (SDDM)
Select Breeze or Breath - nothing else! You will be asked to enter your password to change the SDDM screen.
Then log out, and you should (hopefully) be able to successfully log into a Plasma 6 Wayland session.