What if instead of clients and relays opting in, just users can opt in?

Sure, but I don't see why introduce this unnecessary complexity. In most cases as a client you can pretty much - for whoever you follow or are interested in, request data from any relay, whatever relay on Earth, you really don't care what relay - for cases where you'll get data from people you don't know ("global feed", replies etc), request data from relays that enforce policies you agree with ("no posts without PoW", "all posts must contain the word 'soup'", "only posts from women", "posts must be manually approved" etc)

I think you're both right, but I tend to lean towards the FoaF (friend of a friend) moderation algorithm. FoaF makes newcomers work to get reach, because they need to know someone, and it's something that Scuttlebot did right so we all know how it works. You can include a dial for less toleratant people that can be turned down to 'just friends', and people who want to explore more can dial it up to 3 or 4 hops if they want a global feed.

this feel too complicated. just rely on relay for moderation, right now relay feel likes a data redundancy layer on nostr, but eventually each relay will became its own social community focus on specific topics and interests and acts as a content moderation/curation box that user can plug-in