SDDM KCM provides a few options already, we should also add a way for users to set their preferred virtual keyboard.
At the moment only Maliit is supported. Supporting more than one is a massive technical project, bigger than the scope of what can be requested in a bug report.
This would still be helpful for users who want to select None vs Maliit.
You can already do that from the Virtual Keyboard KCM though. Is the issue that this preference doesn't get synced to SDDM?