Well, thats the thing, if this is an IMAP migration, I dont think you have a choice but to give full access to the mailboxes.
Access to User Mail Boxes in Exchange Online

I just came across an instance where a Microsoft partner was using the recommended Exchange Online to migrate a company's users from Google to Microsoft 365. However, I saw that the person had an option to access Email data for all the users in that organization. There is a actual option to be able to do so. I wonder, why is that option needed? Why would someone who just needs to migrate the email data from Google to Microsoft have access to the users' mails? How can this be prevented while allowing that organization to be able to still migrate the data? Which role should be assigned to them so they don't have access to mails but can still migrate drive and mail data?
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Andy David - MVP 121K Reputation points MVP
2022-04-13T12:24:33.967+00:00 If doing an IMAP migration, its required.
However, are you referring to this? I see nothing here that states full access is required
https://learn.microsoft.com/en-us/exchange/mailbox-migration/automated-migration-neweac