The only place emby needs to write to is /var/lib/emby if you don't change it. By running as root you've probably borked all the perms in it, please give them back to emby:emby and try launching the regular unit again. The full version of the log you posted first would have been nice to understand why it failed in the first place.
If you're feeling adventurous and your systemd (235 and above) is recent enough, you may create the following file:
[Unit] DynamicUser=yes StateDirectory=emby
Then run as root:
systemctl daemon-reload && systemctl restart emby-server
What this will do is assign a random pid to the emby user and automatically chown everything in /var/lib/emby when the unit starts. Note that you may need to userdel emby first, haven't actually tried DynamicUser on existing users before.