I was in this problem few days ago and tried to find any solution but couldn’t find one here i find a solution on my own.
Problem: gnome + gdm + plymouth on >>> KDE + sddm + plymouth sddm not starting and stuck on the logo
Solution 1; disable plymouth x11 sddm will start.
Solution 2; keep enabled plymouth and add this in your configuration
*
services.xserver.displayManager.sddm.wayland.enable = true;