Hi,
Based on your description, I need to confirm with you: What exactly do you mean by free/busy details? Does it refer to busy/free information containing only time or does it include time, subject, location and title?
Based on our test, currently there is no built-in commands export the free/busy information using PowerShell. Currently we could check the free/busy information with Schedule View in Outlook directly. And if you want to view other more details, you may need the full access permission of the room mailbox.
In addition, according to my personal search, I found Microsoft Graph may be able to meet your needs. You could refer to: https://practical365.com/report-room-mailbox-statistics/.Microsoft provides third-party contact information to help you find additional information about this topic. This contact information may change without notice. Microsoft does not guarantee the accuracy of third-party contact information.