I also couldn’t get sddm to work from the KDE settings. I changed the sddm.conf file manually and I’m using the catpuccin theme. The Arch wiki helped (as usual).

You can do this by editing the “issue” file located in /etc/issue, you can put anything in there and it will display in tty login as long as it is text based, if you want it to show date and time or other information here is some possible switches, I recommended Kate if you want ascii because nano can’t display ascii