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.
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.