I would avoid using the server admin if possible. I tried adding the loginmanager server role to the backupAdmin account. Unfortunately restore fails. Looks like it is trying to create the user that is already running the restore.
Failed to restore "dm365-test-2019-07-07-1848" database on "Restoring" step. > Could not import package.
Error SQL72014: .Net SqlClient Data Provider: Msg 15063, Level 16, State 1, Line 1 The login already has an account under a different user name.
Error SQL72045: Script execution error. The executed script:
CREATE USER [backupAdmin] FOR LOGIN [backupAdmin];