In your DNS/registrar settings add Fastmail as a secondary MX, disable DKIM and SPF.
Customize it for your needs (necessary mailboxes/addresses/identities etc).
Switch over primary MX to FM (maybe keeping G as a secondary for a week).
Somewhat optional, but requires some effort - make G send any incoming email to some address not on your domain. You can buy some temp domain and slap it to FM too, it doesn't matter, except you would have two copies of mails for migration time.
After mails start to flow to FM reliably (not from Google) remove G. from MX records completely.
At this point you would know if something is wrong if some mail would be routed through G. instead of FM (by receiving them to that forwading address).
If everything is okay - remove your G. workplace so it has no chance to mess things up.
Before that move your mail from G to FM. If it's only thousands - even connecting Outlook (and maybe Thunderbird?) to both through IMAP and drag and drop mails from one account to another would work. If it's millions... clean up it first. You don't need news on newest sales from 2014.
PS I'm not moved out from my Gmail account but I culled things there HARD. I also moved everything worthy (notably - registrars and mail providers) to my personal domain mail addresses. If I lose it - it wouldn't be that painful anymore.
They do pay us money, that's what the referral fee is.
I was being a bit sarcastic there, though. If anything, new clients => company receives money to stay afloat.
With my own domain being hosted by Google Legacy Workspace or whatever they call it, I've been wondering about this, and wondering if there has there any evidence of Google cutting off an entire domain randomly.
To deal with that possibility I've been thinking what the easiest and most reliable way of providing a "failover" for my domain's mail is.
Has anyone dealt with this successfully?