I have a class that is not part of the UI in which I use with SwiftData. For the fetch method, I pass in the model context and use modelContext.fetch(fetchDescriptor). I also pass in the modelContext to the save method and just use do/try/catch for the insert/update/save calls as I would in a View. It all works fine.

Yes, my architecture for an upcoming app uses inheritance with SwiftData, but I will need CloudKit, so this bug means I will need to figure out a different architecture, unless it's fixed soon.

That's a real shame. Obviously inheritance is a key advantage of OOP. There have been few if any true object databases. Realm was looking good for a while, but Mongo bought them, then dropped support leaving all their users/customers out in the cold. I had great hopes for SwiftData's support for inheritance, but if you can only use it locally, not too useful after all.