Spam filtering on Fastmail has been the same/better than Gmail was - including possibly fewer false-positives on the Fastmail side. Gmail was getting worse about those, both with mailing lists and individuals' emails.
Like many on HN, I'm diversifying my data/access risk across more providers. Too many wake-up calls recently about Google locking people out. There are still some services Google is compellingly better enough that I still use them (Android, Maps, YouTube, Google Sheets), but Email was too precious to tie up with them. I also wanted to finally kick myself into stoping using the `gmail.com` address at all anymore (maybe 20% of my emails before the migration).
One important point here, and I don't know how long you used Fastmail when you tried it, is that Fastmail uses a personalized spam filter. It probably took me six to eight months to receive enough spam on Fastmail to actually train it. (In the interim, they use a non-personalized filter, which as I said, still worked!) Gmail doesn't seem to be able to make personal spam decisions: When I was regularly using Gmail, some types of regular messages would spam-bin no matter how many times I marked them not spam or classified them as a particular category of mail.