This is what I feared.
I looked, and the first tag I found had 2000 compositions in it. I can’t imagine more than 30 of those are genuine. The rest are invented by Roon.
You seem to suggest that “grooming” is a way to find the 30 compositions I actually tagged. But I don’t think that’s reasonable nor practical (if it’s even possibility).
As far as I can see the data is essentially lost.
General point: I hope that much of the tag functionality and structure deprecated in 1.3 will be restored in a later version of Roon.