I did the same, but I never got to like Fastmail so I switched back to Gmail after a year. This time I'm on my own domain though so they can close my account all they want, I'd just setup email with another domain on the same address . Would lose my emails in the process, but that's secondary concern.
I've used a secondary Protonmail account lately and it seems decent. I haven't used it enough to make a fair comparison but if you only gave me 5 seconds to choose I'd instantly choose Protonmail over Fastmail. Same pricing (with custom domain) IIRC.
Edit: Then again as a paying customer I might not be as easily locked out compared to a regular Gmail account? No idea.
I love the experience on Fastmail personally, but you have checked all the boxes to protect yourself if A. You have recent Takeouts or sync of your inbox and B. People know to reach you via the address on your own domain, which you can repoint to a new service if necessary.
If you're doing these two things, you're effectively protected from this issue, even if you use Gmail. Of course, it won't necessarily help you with losses from other Google services you might be cut out of. For an Android user, for instance, there's no real way around potentially losing every app you've ever bought on Google Play.
I've used a secondary Protonmail account lately and it seems decent. I haven't used it enough to make a fair comparison but if you only gave me 5 seconds to choose I'd instantly choose Protonmail over Fastmail. Same pricing (with custom domain) IIRC.
Edit: Then again as a paying customer I might not be as easily locked out compared to a regular Gmail account? No idea.