Ultimately, email clients are services that have maintenance and development costs - if you're not paying for it, someone is selling your information to fund the service.
And, to be clear, I have no stake in Fastmail, nor do I work for them, I am just enamored with their product. I wish I would have made the switch years ago.
They mostly use email for promotion/ad emails and registering for services/games/whatever.
I've thought about switching, but last time I tried I couldn't get my own domain working on the first try, so I just gave up. I think even I would just like to have my own domain in the email and wouldn't gain any other benefits.
How do you get everyone to update their contacts?
Also it seems like even if you forward form Gmail to your Fastmail account Google still knows your business.
I'm not trying to make a case for not doing it but rather would be interested in hearig what has worked for others.
That said, Fastmail does have an import function, but I did not use it when I migrated: https://www.fastmail.com/help/receive/migratemail.html
edit: word choice