I'm a paying Workspace customer, running a domain for my family. I reallllly don't want to migrate off but with each passing month I'm growing more and more nervous.
Apple’s iCloud+ plans (starting at $1/mo) include email hosting with custom domains. I moved my family to that a couple years ago and it’s been fine. See https://support.apple.com/en-us/HT212514
Everyone in my immediate family has 1 or more email addresses in our family domain, and I have a couple of fun/project domains their too, for $0.00 above what I was already paying for iCloud.
Your domains should not be tied to your main Gmail and GDrive anyway. Too many horror stories of wrongful ban leading to a chicken and egg situation around account lockouts.
If it's just for your family, then depending on what you need, you could try OnlyOffice— Either self-hosted, or SAAS. They use MS formats as their first-class native format, IIRC, so compatibility is pretty great.
Nextcloud also advertises an office suite, though I haven't tried it. Nextcloud also apparently integrates with OnlyOffice, though I haven't tried that either— Apparently Nextcloud's own office suite is a rebranding of LibreOffice.
I'd probably go with OnlyOffice, either on its own or integrated into Nextcloud.
Fastmail is really well set up to be a family account provider. You can add multiple domains, aliases to various accounts, and you can mix and match account levels now (kids accounts can be cheaper ones than your own, but still share your domain). Their spam filter works better than Google or Microsoft's, and I set up aliases that deliver to say, my wife and I, so we can both be notified of updates for particular accounts and such.
If you have any issues, a real person will answer your support ticket.
If you only need email, then there are a bunch of providers in Europe that are cheaper (especially compared to something like Fastmail if you need one mailbox for each person in the family), have been around for several years and support custom domains. In no particular order, they are mailfence, mailbox.org, runbox.com and migadu. All of them support IMAP directly (unlike ProtonMail that needs a bridge application and Tutanota that does not support IMAP).
Are you just using it for email? For my family that already had a few Apple devices I switched to using custom domains with iCloud (which is included with any iCloud subscription, even the $1 one).
You can have up to 3 domains configured per family.
Have been a customer for years, great service run by humans.