Depending on your data model, LiveStore is a completely open-source, SQLite based approach for local first sync-y apps: https://livestore.dev/

It's oriented around event sourcing and syncs the events, which get materialized into local table views on clients. It's got pretty slick devtools too.

I did look into it back then, but was not very convenient for my use case. Apart from the data model, I wanted to use Yjs for conflict resolution and wanted more direct control over the sync.

p.s Just wanted to say thank you for all the contribution you do here on HN. Colanode (the app I'm building) is an alternative to Notion and I learned a lot about how you (Notion) build things through reading your comments.