With the way that Apple prevents you from reusing Apple IDs, does it mean that if my Apple ID is blah@mydomainname.com and I migrate mydomainname.com (currently using G Suite free) over to iCloud that I can't set up blah@mydomainname.com?
You can. However, if you switch your Apple ID to a different address before you set up both the domain and the blah@mydomainname.com address to route to your account, you won't be able to set up blah@mydomainname.com for any account under your iCloud subscription for a year.
