You probably know this is not a part of any established camino, so I think that the best way to find out if there is a walking path (other than the 37 direct km alongside the national highway) is to use Wikiloc. It enables me to find tracks for any route that passes through both Chantada and Ourense.
No walking routes came up, but a few bike routes did.
https://www.wikiloc.com/cycling-trails/ourense-cea-chantada-escairon-ourense-18971320
This is a circular route so it has two options. It looks like you could go to Cea on the Sanabrés and then back down to Ourense, but that would involve backtracking, because the next day the Sanabrés will take you from Ourense to Cea. The other half of that circle starts in Escairón on the Invierno but then you would miss Belesar and crossing the Miño, which would be a shame, IMHO.
Here's another circle on different roads
Ourense-coles-chantada-peares-ourense Road Bike trail in Las Caldas, Galicia (España). Download its GPS track and follow the route on a map. Record your own itinerary from the Wikiloc app, upload the trail and share it with the community.
www.wikiloc.com
The other thing would be to look at those wikiloc maps to try to find a place to stay in the middle. The straight shot on the national road is 37 km so maybe you could do that in a day, but all of these biking routes take less traveled roads and that makes the distances longer.
Good luck with this, I'd be interested to know what you decide.