How should an offline-first mobile app handle data synchronization?

Hey folks, I'm working on a mobile app and need a way for users to reliably access data even when their internet connection is spotty or completely gone. I'm trying to figure out the best way to sync up the data once the device gets back online. Should I go with a client-side diffing method, just use timestamps and last-modified dates, or maybe something more complicated like operational transformation? What do you all think works best and is easiest to keep up with for offline-first sync?