Just found out, that SDDM running on wayland in TTY1, after logging in, still opens a new wayland (plasma) session on TTY2… Destroying my dream of a smooth login effect. 
I asked chatgtp (i know) how to solve this, and it stated that I could add
SessionCommand=/usr/bin/startplasma-wayland
SessionCommandFallback=/usr/bin/startplasma-wayland
to the [General] section to force it in current tty. But this had no success.