Nice! I agree wot and some pow seem to be the most effective solutions (seeing captcha as one form of it?). Right now the app shows a feed based on friends of friends but I think the replies are not covered by that. But I may be wrong here and nostr:npub16zsllwrkrwt5emz2805vhjewj6nsjrw0ge0latyrn2jv5gxf5k0q5l92l7 knows better about the iOS internals.
We do have backend services yes, in fact we have now a server to calculate followers that is internally using neo4j and I think it has potential to extend the classical friend of friends Wot to a page rank based solution.
https://github.com/planetary-social/nos-followers
Pagerank could extend it to more people than the one level friend of friends. We are calculating it currently but we are not using it for the moment