Zero knowledge proof android/iPhone app integration is the idea. Many apps integrate with your default calendar. Not all deserve to know 'everything'.

Example: map app wants to remind you about an event and to set a route. It doesn't need anything else. Allow that app time/date and location, but only before the event through a notification, or pre-authorization.

This may require a false 'default' calendar of sorts, where you share those parts of events with it. Every app should see only what it needs though. (Multiple defaults? I'm not a dev unfortunately)