I don't understand your claim, with fly.io it seems like you attach some storage to your application and store the SQLite file there, semi-locally, much the sam

I don't understand your claim, with fly.io it seems like you attach some storage to your application and store the SQLite file there, semi-locally, much the same as any other use of SQLite. The SQLite database code is still inside your chosen application, it's just loading the file from some "persistent volume" abstraction.