Something to bear in mind: when you use one spam filter for a while, it learns the sort of spam you get and will do a better job. When you switch to a new provider with a new spam filter, you have to train the new spam filter before it can be as good as the provider you left. (FastMail enables personal spam filters for improved results when it gets to 200 spam and 200 non-spam messages.)

Some find FastMail’s spam filtering better than Gmail’s, some worse.

I had that problem, and fixed it by doing these four things:

* Train the built in Bayesian filter fully * Setup SPF * Setup DKIM * Point MX records at Fastmail directly (rather than forwarding via some other service)

More of a pain than GMail which often "just works", but still relatively simple to setup. I suspect ultimately more robust than using mail forwarding on consumer GMail (the rules of which constantly change and it gets less powerful, if you don't buy GSuite and point your whole domain at it).

This is also merely anecdotal, but I don't recall having ever received more than a few spam emails in either my Gmail or Fastmail inboxes.