Neat, I just put together my first MacOS app. Am thinking about an iOS version and using the Apple data thing to keep them in sync. Any tips?

https://github.com/agoodway/goodday

CloudKit makes it easy - and you can send push notifications (all part of CloudKit, no push tokens or subscriptions to manage) to inform the "other" clients when there is new data to retrieve. Best to think about each task being an individual file in CloudKit.

Generally speaking it just works.