First question I have is are there any features in the migration software that make this easier than manually combining all repositories before migrating?

Otherwise I assume the approach is to export and import all documents into one repository on-prem first, then recreate all the users and security. Now migrate to Cloud as normal.

The security would be the most difficult part without a way to map it over.

This is why I am wondering if the migration utility to Cloud has any features to successively import repositories with the user mapping feature. Then we could just create all the users in the new cloud system and run the import against each repository.