I mention that because Fastmail for sure has a calendar, and don't get me wrong, it's no Google Calendar by a mile, but it seems "sane"
Letting my assistants see and edit my contacts. Alternatively, sharing with write privileges.
Either way, I'm very confident if you ask them they'd be glad to see if they have something that would meet your needs: https://www.fastmail.com/support/
for clarity, I don't work for them or get a cut, I just like their product bunches
I mention that because Fastmail for sure has a calendar, and don't get me wrong, it's no Google Calendar by a mile, but it seems "sane"