Suggestion, use both
I don't know what problems you might encounter moving from Gmail to Fastmail, the only thing that you might encounter is Gmail allows assigning multiple Labels to an email while in Fastmail you don't have labels and can only put the email into a single Folder.
I use both Gmail and Fastmail and suggest you try to use both. They are both excellent services. If you keep Gmail make sure you take advantage of all their account recovery measures as you don't have any Google support that you can call on like Fastmail offers.
Cheers... Russ
|