DB sync between SQLite and PostgreSQL (Java-Flutter)

I'm creating Java/Flutter app using PostgreSQL for server side and SQLite as local DB for client side, how can I sync those DBs effectively, so the app works both online & offline?

I found something about Powersync, a Postgres<>SQLite sync layer. But if I understood it correctly, it would send data to PostgreSQL instead of my Java API.

Is there any way to sync it but also use backend API?

I'm in the PowerSync team. With PowerSync, downloads go "directly" from Postgres to SQLite. But for writes, PowerSync doesn't write anything directly against your Postgres database - you can write the changes using your Java APIs. It should work well for that use case.