I think it sounds like a good solution, although that brings another issue to the table, that being the enormous amount of communities with dead administrators on Lemmy, which will make everything harder.

So all their posts/comments are hidden from everyone, until an admin manually approves? It sounds like a good option, but would require a bunch of work to implement. Mostly frontend changes for the review, on the backend it mainly needs a new user column bool reviewed_by_admin.

It’s a horrible system for new users. It would kill growth in an instant.

Spam is only spam when there’s a number of substantively similar posts. A first post doesn’t meet that test, by definition.