@julian yeah, its in the [unit] field in After= ... just add mongod.service and that should help, but it is not really needed.
It will keep restarting until it is working... if the service has been enabled
I tested it out a few weeks ago. One server has it, the other doesn't have it and both took the same amount of time, unfortunately. The reason is, mongod will report as started/alive/online... but the other dependencies did not start yet (one way to test, is to stop the mongod service, start it up again and immediately try to access the DB... mongod will report an error for a minute or 2, despite reporting that the service has started)