KAuth is designed to perform specific tasks as root, ideally as small as possible. Starting a full-blown application and keeping it running is not among its intended use cases.
Not sure what exact things are missing - it could be as simple as all of the KDE/XDG/DBUS/WAYLAND/XAUTHORITY/etc. environment variables that SDDM will set for processes in the session but that won’t exist for a KAuth helper binary started by systemd directly. You could try to figure it out, but it’s not something that anyone in KDE wants you to do, so you’re probably on your own if you go down that route.
If it’s about editing files outside of your home directory, consider using Kate running as your regular user and opening files via kio-admin instead.