iMac Documents folder empty but storage info shows it contains 200gb of files

Check the System/Apple ID/iCloud/iCloud Drive preference pane to see if iCloud Drive has been activated and if (under the Options button) if Documents and Desktop has been included. If so then you'll need to select iCloud Drive in Finder's Sidebar preference pane and the click on iCloud Drive to see what's in the iCD:

Sometime these items get turned on during an upgrade or update.

Where are you looking to get usage data? The definitive view is using Disk Utility.

Do you maybe have more than one Documents Folder? One you created yourself and are looking in the wrong folder?

The default Documents folder will be under your user name on the internal drive.