The updater service must run under the system account. Otherwise it wouldn’t be able to perform the update without requesting an admin password.
If you customized your installation that much, I wonder if you need this service at all. It’s realyl only needed to perform a server update from within the Plex app. If you manage updates yourself, you won’t need it anyway.
The service is running under an administrative user, just not the system account. So it doesn’t bring up an admin prompt when updating.
This is because I have Plex running as that same user, not the system account, because otherwise it can’t access files on a network share.
I also don’t want it to start automatically at login, because I have something that starts it after a delay, to ensure the network drive has mounted first.
I can’t follow this logic. It is certainly possible to give a non-admin user account access permissions to all kinds of network shares.
But that doesn’t really matter. This service doesn’t need any server access permissions. It is only needed and uses in the case of a software update of Plex server.
It is totally independent of the user account under which you are running Plex server itself.