> The experience on iOS is similarly frustrating.

I agree with "frustrating," but it is solvable without 3rd-party software.

https://www.fastmail.help/hc/en-us/articles/1500000279941-Se...

You can either use their "Device profile" or do it manually.

I don’t have any complains with contact management on iOS with Fastmail. Apple’s CardDAV and CalDAV implementations are way better than they used to be. What issues are you seeing?

How's that possible? It seems like Whatsapp integrates seamlessly with my contacts