Native integration with macOS/iOS calendar and contacts. This has the benefit of being a single integration (the app will just ask for access) and can then grab all data, regardless of storage. So for example, I have two personal calendars and three work calendars. Setting up 5 integrations for this is a hassle, but just pulling data from the macOS app is much easier. Fits with your native strategy + end-to-end encryption approach (no data passing your DC under this scenario).

Same for contacts.