Apart from paying £2k a year for a tool like ShareGate, what is the easiest way to copy content from one tenant to another?
I would suggest our tool DMS-Shuttle for SharePoint. You can download the content (entire structure from document libraries) to your file system and then upload this content from the file system to another office 365 tenant. After that you can use bulk update for setting metadata, if needed. There is trial for 15 days http://www.dms-shuttle.com/downloads
