Re: spam, this is why you should be using your own domain with wildcard alises. If an alias becomes a target of spam, you can block all mail coming to that alias.
Re: calendars, with Fastmail you can use any calendar you want. I use iCloud for my calendars and just connect it to my Fastmail account.
Yes, I can vouch for this, I have had several shared calendars using fastmail. I use the Mac/iOS calendars daily.
One caveat is that Google calendar can’t subscribe read+write to an iCalendar feed. That’s their problem though. All other clients are fine, including outlook IIRC.
I've been doing the wildcard/per-site email thing for a few years now, unfortunately these spam emails are coming to a more generic address that I used to use for everything that I can't quite ditch yet.
Also regarding calendars, can you share that calendar with arbitrary people who don't have Fastmail accounts?
Re: calendars, with Fastmail you can use any calendar you want. I use iCloud for my calendars and just connect it to my Fastmail account.