I ran into the same issue and was able to work around it by running a few iterations of apt dist-upgrade and autoremove which seemed to fix the dependency issues and was able to get the system booting to the proper login screen again.
Thanks!
autoremove before installing plasma-desktop, and then dist-upgrade and another auto-remove after plasma-desktop-data worked for me
Just replying because I fixed this issue another way. I’m running Arch 6.9.5 w/ a Nvidia RTX 3060 Ti.
I had to add this to my /etc/modprobe.d/nvidia.conf.
options nvidia-drm modeset=1
options nvidia_drm fbdev=1