With more at home pandemic time I decided to just do it and have been super impressed. The Fastmail docs are really good, setting up DNS to use my custom domain had explicit steps and pictures for my specific provider (gandi.net) along with a ton of others.
They have super easy to setup import from Gmail to get things over initially and to keep things coming as you convert accounts.
They have really good instructions for all IMAP clients and it's been painless to set those up.
I did have to spend a day going through every account in 1Password and changing emails, but the super simple aliases you can setup in Fastmail make this great. I have a bunch of different ones for different types of sites (some very specific, some general catch-alls) on my domain which I can now blacklist in the future if I need to.
The main issue is that you realize a lot of sites are just terrible, many don't let you change your email at all and require contacting support. Some don't even let you change after contacting support (postmates just says to 'create a new account under your new email and abandon the old one').
In general though I'm happy I did it and the Fastmail site has been pretty impressive. I like paying for software I think is high quality and valuable (where the business and customer have aligned incentives).