Hi,
I was researching this and found this page: Utils – Mp3tag Documentation
This function removes unsupported fields, i.e., metadata entries present in the files that have no visual representation in Mp3tag. The fields are permanently removed.
But, I cannot seem to find such thing in Mac version?
In fact, I cannot even find such things as Utils?
It’s currently not yet available in the Mac version. You can simply remove the Tags there and undo the change to have only supported tags written back to the file.
By removing you mean following flow?:
Seems quite risky, when something unexpected happens during this process, you may end up loosing all tags. 
You decide, and quite frankly, this applies to other operations as well.
A backup reduces the risk quite significantly.
Yes, separate backup is a must (backing up around 2TB is a challenge though), but is that flow itself correct?
Yes, undo would write back only the metadata that was previously displayed in Mp3tag based on Settings → Tag Types.