If you have your own domain, Fastmail banning you is a minor speed bump at worst. You can just point your domain at a different email provider.
You can similarly transfer domains between registrars if one of them doesn't like you. And in practice, your domain registrar not liking you is pretty rare; I've only heard of it happening with famously evil alt-right websites and they generally manage to find a registrar that will take their business.
