I find myself going back to your NostrPost and seeing if the relay implementation over there can run as an Android app. We just need a database.
NostrPostr was my rookie attempt at doing this while getting to know nostr and I gave up when I ran into some concurrency issues. It worked locally when testing but not when it got hammered by multiple clients. But for a local process on your phone, maybe that's not even an immediate issue. Fingers crossed.
rookies rule the world.