This looks really great. Have you thought about adding Operational Transform or CRDT primitives to your offering? Realtime subscriptions are nice - but they're even better with realtime collaboration. Because you're using SQLite directly, it's easy(er) for you to build in efficient CRDT mutations to your database rows because SQLite's BLOB column supports incremental IO: https://www.sqlite.org/c3ref/blob_open.html

This could allow you to save a CRDT update operation without needing to read the entire CRDT blob into memory, parse it, and then save it all back to disk.