Fastmail's calendar works reasonably well. My two complaints with it:

- There isn't a convenient calendar widget; Google's calendar widget only works with Google's calendar. I'd like something exactly like Google's calendar widget but working with Fastmail's calendar.

- Sites that integrate with Google Calendar but not with arbitrary CalDAV servers.

I could live without the latter, but the former is a dealbreaker; I'd switch given a functional widget that is fully self-contained and doesn't require some separate sync app.

Google Calendar is pretty cross-compatible.

https://en.wikipedia.org/wiki/ICalendar

Huh? There is little to no lock-in