Does qua de re mean something like “on this topic”?
That seems to fit this example:
Qua de re cum sit inter doctissimos summa dissension . . .
It certainly looks like a valid reading of it. Qua de re = de qua re = de hoc/illo, and “de” can certainly mean “about”, so I don’t see why not.
res can mean just about anything, but here “topic” would be a good translation: “on this topic there is extreme disagreement among the specialists”