Shameless self plug, but my workout tracking app[1] uses a sync engine and it has drastically simplified the complexities of things like retry logic, intermittent connectivity loss, ability to work offline etc.

Luckily this is a use case where conflict resolution is pretty straightforward (only you can update your workout data, and Last Write Wins)

[1] https://apps.apple.com/us/app/titan-workout-tracker/id644949...