I think I can clarify this. Let's summarise what we all should know:
Open Web UI needs to create a user automatically as admin. Even if you use Cloudron for authentication, this user will only have user rights. You need to log in using the account with the demo credentials and give your Cloudron account admin rights. There is no way around this account.
If you decide to use the automatically created account, you cannot change its e-mail address, at least not using the GUI. It probably could be done in the database.
If you want to delete this automatically created admin user, you don't need external software. Within the cloudron app settings, you can go into the terminal and connect to postgres, then there is a user table.
This is probably the best approach.