Some thoughts…
If you create a new user (via Plasma X11), can you get a working Plasma (Wayland) session logging in as the new user?
Eliminate the display manager as a culprit by trying SDDM. Same behaviour? With SDDM at least, Wayland sessions are logged to ~/.local/share/sddm/wayland-session.log (but I’m not sure where GDM does this).