I recommend everyone does this; not just if you switch to fastmail. Having your email identity tied to a particular service provider is a terrible idea. Email will probably be with us until we die; and chances are very high you won't want that particular email service provider for 60 consecutive years.
I used gmail with a custom domain for years before I finally decided to move to fastmail, which made the move pretty painless. That said, when I set it up gsuite with a custom domain was free. I don't think thats the case any more.