Yeah, never really saw the use case of private NIP-29 groups, when private relay groups are so much simpler and clearer in intention. Either I want something public, or I don't.

If I am concerned about privacy, my first thought wouldn't be to store anything at all, on anyone else's relay. Can't even confirm deletes, that way, or control who can watch the traffic.

I'd want my own relay, and to have encryption on top. And, if I were really serious, I'd put the relay on my own server, on a private network or behind Tor, or whatnot. I'd engage the entire architecture. Lock it, all the way down.