Very cool overall but the sync->async change means you can't always use it as a drop-in replacement. I assume it's making network calls in those cases which will take significantly longer that local writes. I wonder if something could be done so that it write the data locally and kicks off something async in the background to sync it up to the backend. Maybe with just a timestamp in case you go offline to handle conflicts (or even way to register a callback if there is a conflict so you can decide how to handle it). I know that all makes this more complicated but I'd be worried about adding waiting for a network call where I currently do localStorage calls.

But still a very cool project, thank you for sharing!