Mistaken
We do not who exactly you spoke with but "The tech also suggested wiping the drive, reloading MacOS, and then restoring everything with Time Machine "
The above would put the computer back into the same condition is if now
The category System Data is a collection of files which include:
• System temporary files
• macOS system folders
• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)
• Personal user data
• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)
• Cache files
• Fonts, plugins, extensions
• Other files that are not recognized by a Spotlight search
• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package
•Time Machine Backup Snapshots that have not been transferred to the TM Backup Drive
They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders. You can use either of these two free apps, GrandPerspective , to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.
