Earlier quoted context omitted.
It's probably worth using your own domain even when using third party email providers, so you are in control of your email address.
That's a good point. I don't suppose there a way of leaving Gmail and bringing your @gmail.com address with you, is there. That was one of the changes that was forced on phone companies years ago to make it easier to move provider.
I also use a lot of the other fastmail features, like mail aliases, DNS, and file storage and web site serving. I'm very happy to pay the money.