That's a lot of posts in 6 months. Nearly 3 a day every day.
You can certainly do one or other criteria with the auto groups extension. Not sure if you can do both, have a try.
https://www.phpbb.com/customise/db/exte ... to_groups/
In group forum permissions, set the registered users and guest groups to no access and then set the new group you make to standard access.
No one will be able to see it unless they're in that new group.