Even though I have no expectation that my emails are 100% private, I'm still incredibly hesitent to trust any company operating out of Australia with their horrible stance towards encryption and backdoors (which is not Fastmails fault of course, it's a pretty horrible situation for them).

I'm fairly sure that Migadu also supports caldav, although I don't think there's any UI for it on the web. I think there's pretty much an expectation that most people using Migadu are bringing their own clients rather than using the web one anyways.