See if you can get into a Plasma X11 session by running startx from the CLI. If successful, open System Settings & change your SDDM login theme to “Breeze”. You should also change your KDE Global Theme and other appearance settings to “Breeze”, as they will definitely work with Plasma 6.
Then log out of the Plasma X11 session & see if you can now log into a Plasma Wayland session.