This initial design is to focus only on events that need read protection. I get generically the idea of everyone running their own inbox relay that accepts all the posts tagged to them but that is not what I’m trying to achieve here and I don’t see what advantage there is to reading a kind 1 from this relay vs any public one.
All REQs require AUTH but writing events does not. As long as the tagged user in the event has an active inbox, the event will be written to the relay. Since as you mentioned no content is moderated in this approach, I added a special use of kind 1984s to treat spam reports as delete requests that are automatically honored by the relay as long as the user reporting the event is the user tagged in the event. This allows you to treat the relay like a personal inbox and simply delete messages “sent” to you even though you are not the author.