In a very next future I will create a whole new NOSTR client acting as a NOSTR underprotocol, focused in privacy on your ZAP micropayments.
Metadata will be private on yours, neither my database or the Bitcoin network will have your metadata.
I cannot share all the specs of the underprotocol by now, but for you all that live in totalitarian regimes, that doesn’t follow the ideals of freedom and individual digital sovereignty (because they believe the State is sovereign), will be well assured there.
Your keys are your keys, your metadata are always your metadata, be aware to not sharing it with anyone.
The real sovereignty starts from the individual, always!