So say your custom domain is example.com and you have a user who logs in with me@example.com that has data in Google Gmail, Drive, Photos and Calendar. What are the migration steps? I'm finding this really confusing.
Backup Gmail and Calendar, switch to business essentials plan, then cloud identity plan, set up email for me@example.com with some other provider and you can still use me@example.com to access your old Drive and Photos?
Yeah, those steps seems about right.
But you'd probably want to set up the new provider as your first step and upgrade from the legacy free edition as your last step.
I also like to point the MX records to the new provider before doing the backup so there will be no incoming messages while the backup is in progress.
Disclaimer: i haven't followed the steps myself since i'm paying for a personal Workspace account since a few years ago.
Info about Cloud Identity Free: https://support.google.com/cloudidentity/answer/7319251
Steps for adding Cloud Identity Free: https://support.google.com/cloudidentity/topic/7555155
You will only lose access to Gmail and Calendar, though make sure you migrate email and calendar before moving to Cloud Identity Free.
Disclaimer: i haven't followed the steps myself since i'm paying for a personal Workspace account since a few years ago.