Two way sync seems like it's asking for trouble. What do you do when an event is modified on both sides, etc. Even if you don't plan to modify things on both sides, you'll have it done for you when daylight savings rules change, assuming you're subject to DST.

I'm not familiar with Apple's calendar services, but Google and Fastmail both let you publish and subscribe to calendars; if you need to see everything everywhere, then I'd cross subscribe on the services. If you need to modify something, then you've got to do it at the source of truth.

Personally, I'm on Android and Google never built Caldav support into the built-in calendar, so I just put everything in their calendar. I might prefer to use Fastmail, but I don't care enough to get a calendar client.