One related issue is how to get email off of Google Workspace. After checking out various options, I've settled on using imapsync (https://github.com/imapsync/imapsync).
Unfortunately, the official Docker image was throwing errors for me while trying to move an almost 100k email account. Ended up building a modified version which I have now been able to successfully use to migrate. Image: https://hub.docker.com/r/jauderho/imapsync Dockerfile: https://github.com/jauderho/dockerfiles/blob/main/imapsync/D...
HTH.
Protip for those moving to iCloud+ custom email domains, you HAVE to temporarily set the SPF record to exactly match what Apple wants to see during the initial setup. Else it will complain that setup cannot be completed. You can revert it to whatever you had it set to previously (plus include:icloud.com) once the setup is complete.