Haven’t looked at this post in depth but I think I had a similar issue. Ultimately my solution was to edit /etc/sddm.conf.d/wayland.conf, and make sure these settings are set (not replace file contents with this, but you should be able to find these in the file):
[General]
DisplayServer=wayland
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell
[Wayland]
CompositorCommand=kwin_wayland --drm
The gist of my understanding was there is some sort of issue with the handoff from SDDM’s X11 greeter to KDE (SDDM uses X11 by default, apparently, even if you use Wayland for the main DE).