Why not just use DMs?
Discussion
Because the nostr clientsare retarded. Ox is a current messaging for Android and ios. Key maybe slightly better
Amethyst is okay. Though it creates 2 notifications for 1 DM. Which is really annoying.
Nostr DMs are currently not at all secure. But there is a proposal to bring Signal-like encryption to them (it will still be up to Nostr clients to implement that)