check this out https://developer.apple.com/documentation/mediaplayer/mpmusicplayerapplicationcontroller/2815055-perform

I can only say that, years ago I tried to add GameKit to a non-gaming app and it was ultimately rejected (thankfully I hadn't burned too much time on that feature). You'd probably do better to integrate with HealthKit, allowing users to contribute to their activity rings.

Same. My production app works fine on iOS 14, missing "Add People" on iOS 15. Compiling with latest Xcode doesn't help. Also filed a report (FB9342748) during beta. Apple's first-party apps like Notes and Reminders doesn't seem to have this issue.

is the device storage full?

you need to import CloudKit for CKRecord. I hit a few of these as well, seems like Xcode 12 had some implicit imports. Just make the imports explicit, and you're good to go.

CoreData. The answer is always CoreData. And if you're planning for sync, just enable CoreData-CloudKit from the start, because migrations after-the-fact won't be easy.

we're only getting a glimpse into your code, but my guess is that, when declared as vars, they are instantiated when the struct/object is created and don't have access to the environment variables such as "managedObjectContext". you might try using lazy?

check out https://github.com/jeffreykuiken/CloudKitSyncMonitor for hints at how to determine when first sync completes. You could also explore using multiple persistent stores, one with a read-only baseline of data, and another that syncs from CloudKit any additional data beyond baseline.

short answer: don't worry about it. Apple has invested heavily in CoreData and uses it extensively. wrt deleted records, Core Data will clean up its storage over time. longer answer: to understand what's going on, you'll have to dive into the architecture and implementation of structured database systems, and specifically how SQLite (which is used by default by Core Data) organizes data and indices into its file structures. For some insights, see here https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CoreData/PersistentStoreFeatures.html

turn on all these flags and see if the debugger points you to an issue in your code. https://useyourloaf.com/blog/debugging-core-data/

This sounds related to the current issue that support for DiffableDataSource in FetchedResultsController doesn't handle updates, only inserts, deletes, and moves. FetchedResultsController is the backbone of @FetchRequest in SwiftUI. See https://www.avanderlee.com/swift/diffable-data-sources-core-data/ for a good discussion of the issue.

Hmm

Do you have a Searchable index on favoriteColors?

It appears that subscribing to the public database is deprecated. CoreData's support for CloudKit public database does polling. See https://developer.apple.com/videos/play/wwdc2020/10650/ with a reference at the 7:30 mark, and a more-detailed discussion starting at 10:20. I would recommend the entire video, even if you're rolling your own CloudKit implementation, as it has lots of insights into how achieve sync without the tombstone magic in CKFetchRecordsZoneChangesOperation.

Create a test account to do your development and testing.