I considered fastmail. Their integration with android contacts is terrible,you have to maintain contacts separately.
I can do that,but it doesn't work for non technical users of my family.
It'll do bi-directional sync between Android contacts/calendar and Fastmail (or any other CardDav/CalDav server)
Also available on F-Droid: https://f-droid.org/en/packages/at.bitfire.davdroid/
https://www.fastmail.help/hc/en-us/articles/1500000279881-Se...
Android does not support CalDAV or CardDAV, which are used to sync calendars and contacts. However, a workaround is installing a CalDAV or CardDAV sync adapter.
We have tested and recommend DAVx⁵, which is approximately $5. Once you have added your account in DAVx⁵, you can set up calendars or contacts in the app of your choice, and the changes will sync with Fastmail. > 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.