I'm sorry. That's not easily going to happen. Even if you decided to run your own relay and implement a policy to nuke database entries after 30 days, anyone on Nostr can still rebroadcast your notes to any existing relay. You could submit a request to have that event data to be deleted, but there's no guarantee that it will be honored.

Your best bet would be to make a new npub every 30 days. Though, doing this, you sacrifice your identity and web of trust.