I will also second Fastmail as a Google alternative. Does what I need well.
I have encountered a couple minor bugs with the calendar but nothing I would classify as a show-stopper.
First, once in a while when you load the calendar page an event doesn't show up. Refeshing fixes it. Never had issues with events not showing up on mobile, though.
Second, I sync external calendars and sometimes events show up as also being on the external calendar. Weird, but easy enough to ignore.
I personally found that events showing up on the external calendar was much worse than just weird. Every job interview cycle I've gone through using Fastmail has had its own new issue.
$500/mo < $720/mo… but not near enough cheaper for kids email IMO