That road is called Dol-y-Moch according to the list here: https://www.sabre-roads.org.uk/wiki/...Roads_in_Wales . It is indeed a great ride. I particularly liked the view of Moel Llyfnant: on the Arenig Fawr race (which I ran the day before cycling over Dol-y-Moch) you just go up and down a steep, grassy slope on the north side of the mountain, but from the south it's a much more dramatic scree slope.
I have: on a very hot day last Summer, I had finished my water on the walk up Aran Fawddwy from Llanuwchllyn, and descended where the crags end on the ridge to Drysgol, to refill my bottles from the headwaters above Creiglyn Dyfi.