Is there any reason to switch to fastmail from an Microsoft 365 business account? Microsoft offers mail, 1tb cloud storage and more for essentially the same price.
Theoretically you might save a bit with Fastmail - it seems like you can have 1 standard account that is $5/month and owns the domain and then other basic accounts at $3/month that have an alias for the domain, but I don't know how that works in practice. For a large family the savings could add up.