Questions not directly related to kde but it's a kde app.
I'm still looking at running different desktops on a per user basis and unlike others sddm seems to have what's needed. According to the arch wiki this can be spec'd per user name.
This is done in /etc/sddm.conf however there is more in /usr/share/sddm. That seems to be where faces and themes are spec'd. Also facilities to run scripts before log in and after log out. I have added another user but there doesn't seen to be any way of having different scripts run for different users as nothing has changed in usr/share/sddm/.
Maybe I am missing something but given that different users can use different desktops it should be possible to have differing scripts. That may be needed for other reasons as well.