Earlier quoted context omitted.
I totally get that this is largely about losing a decade of "purchased" stuff that is tied to the email address, meaning you're being heavily steered into starting paying for emails etc just to keep your purchased movies, apps, and so on. If you do decide to move away it may not be as bad as you expect for the everyday email, contacts, and calendar. In particular when it comes to your email you probably don't need to…
I have a number of users at user1/user2/etc@mydomain.com on G Suite. If I started to use Fastmail, could they all continue to use their existing email addresses at Fastmail to send/receive mail, access their historical emails (if I copy those across) etc using whatever client? And second question: if after this G Suite shutdown in May (or whenever) Gmail is stopped for these users they can presumably still link Fastm…
> I have a number of users at user1/user2/etc@mydomain.com on G Suite. If I started to use Fastmail, could they all continue to use their existing email addresses at Fastmail to send/receive mail, access their historical emails (if I copy those across) etc using whatever client?
Fastmail supports multiple users per domain (and multiple domains btw). For those users it should only need the client updating with details of Fastmail's servers instead of Google's. Then it should work the same.
> And second question: if after this G Suite shutdown in May (or whenever) Gmail is stopped for these users they can presumably still link Fastmail to Gmail? Or would they have to use some other email client?
You'd not be using the GMail app at that point. You would either be using Fastmail's own app or a third party client such as the Outlook app (which is actually very good, only moderately tied to MS, and works with other email providers).
If your users are using GMail on the web they'd switch to the Fastmail site instead and, personally, I think that wipes the floor with the mess that GMail has become.
> Would they each get their own credentials for logging in or would I have to trust them with my "main" Fastmail credentials? I've never used another email provider before.
From Fastmail's user configuration screen: "Each user has their own inbox and login".
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Whatever and whoever you choose, try it first whilst you've still got time. Pick up a really cheap domain name (a random one as it is a throwaway for testing) and start a trial account. Configure a few users, set up the app, and try it out.
Transferring emails is quick with Fastmail, but I wouldn't do that for any kind of test as it is very easy to get a setting wrong and move them from GMail instead of copy them, which makes your trial somewhat more permanent unless you move them back.