i'm implementing the change to a compact event format that exploits the pubkey, p and e tags being references. it also has a "placeholder" code for events that refer to events not yet in the database, these store the whole event id.
idk what to do about sharing the access to the relay, i think android has some features that would allow you to bind to the websocket listener and plug it into an IPC interface for other apps to also use, would require you to stipulate that the battery permissions on the app be active, so it could be a standalone service. idk about other users, since that rarely would be needed concurrently it can just be a startup service, like how orbot and wireguard work.
i'd put the encryption side of it at the OS level, just encrypt the whole app specific storage in the user's profile. storing drafts i think you could then eliminate the encryption step for when you are using the already encrypted db file storage area
as for the DMs storage, probably better to just put that in a separate place inside the encrypted file storage where the database engine reads and writes, so make two subfolders for that.