for completely private note storage on relays you could giftwrap with a p tag = H(nsec+”storage”). You could pull these down, unwrap them and store them in nostrdb for local querying.

giftwrapped notes have no internal signature, so your private, unwrapped data could never be broadcast to relays.

the wrap itself has no publicly identifiable information tied to you, so people don’t even know who is storing the data.