Probably with SQLite and like a local in-app relay

The Dipity side is for the dev to answer, but on the Reticulum half: Reticulum itself doesn't cache for offline people. That's LXMF, the messaging layer on top. LXMF "propagation nodes" store encrypted messages for recipients who aren't reachable, peer with each other and sync, so the recipient can pull their mail from any propagation node later. While stored, a message is tracked by a transient ID because the node can't read it. Anyone can run one with lxmd -p (from the lxmf package). The key difference from a pure Bluetooth mesh: if every device is offline, nothing moves. You need at least one always-on node holding messages.