Did you look into https://ditto.live/? Its and off-line first db too. I never used it but looks very interesting and would be interested how it compare to the technologies you picked

I love RethinkDB. I really believe it is a database that deserves more attention. We've been running on it for six years and it is flawless, easy to use. Like any other database you need to understand how it performs and how to query efficiently.

But it does has everything required for streaming and syncing data in a modern way. You can open streaming queries and even with a backlog of data, so syncing should be pretty easy. Maybe would be feasible to fork PouchDB to use RethinkDB as a backend?

Firestore isn't really a client side database then either. It is certainly also querying results from a server.

I have a question, how do you manage the fact that Supabase is based on a relational db[1]? Do you just put all fields (except the primary key) in a JSON column[2]? Does it work well if you use it that way? We were investigating it as a way to escape Firebase's vendor lock-in but the fact that we would have to manage schema migrations was a bit of a deal breaker based on our use case. I'm also interested in hearing about any alternatives to Supabase that use a document-based nosql db.

[1] https://supabase.io/database

[2] https://www.postgresql.org/docs/13/datatype-json.html