The migration process was pretty painless. I did a one time import of my historical mail from gmail and then set up an autoforwarder --> specific fastmail folder that I look at from time to time and either unsubscribe from whatever sent an email there or update the address as appropriate.
You could split the zip archive into smaller sized volumes, e.g. 45MB, and store those, assuming it's something you don't access too often.
Great web and Android apps, simple and fast. I contacted their support once and wasn't disappointed.
No issues at all until last week: they lost an incoming email (which was actually spam, as they could share with me the subject and sender info) due to a bug during an upgrade on their side. They credited my account with US $5 as apology.
One thing that hasn’t been a problem but which causes me a little anxiety is that with GSuite I felt I had basically infinite storage; I sorta wish Fastmail had a tier that was a “you don’t ever have to worry about storage space” type thing. Maybe I’m worrying for nothing and I’ll never even get close to their limits.
The only trick is you do have to configure configure the addresses if you want to send (not receive) mail from $recipient@your-domain.com.
I use the standard plan.
[1] https://jmap.io/
Aside from that, I have no complaints about their service.
One of the be things about fastmail is they've been rolling out noticeable improvements to their web apps over the last 2 years.
I'd say the downsides, at this point, are the labels model for emails is slightly different than gmail. If you've decided you really like how labels work, Fastmail's don't work quite the same. And their contacts app isn't good. Though given the pace of improvements elsewhere, I suspect they're working on a fully-featured contacts app.