Hi Elliott,
According to your description, I understand that you want to change the default calendar permission(the default setting is Free/Busy Time) for every users in your environment.
If I misunderstand your concern, please do not hesitate to let me know.
Please try to run following command to achieve your goal:
Foreach ($Mailbox in (Get-Mailbox -ResultSize Unlimited)) {Set-MailboxFolderPermission -identity "$($Mailbox.Name):\Calendar" -AccessRights LimitedDetails -User Default}
Use the following table to match Microsoft Outlook permissions with the access permissions that Microsoft Exchange Server uses:
Best Regards,
Allen Wang