The Sustrans path goes to Steiner, so you could ride that, then hop off straight onto the back road from Steiner to Buckfastleigh (Colston Road) from there, or come off that path back up the hill a bit at Huxams Cross, and go up the Rattery road to Willing Cross, then along the Pennywell lane, and down past the old race course field.
There’s a couple of greenlanes that just run between Colston Road and the road along the ridge (Willing Cross / Pennywell lane), but that’s it that side. Not ridden either for a while too, so dunno what they are like now…
The other side of the valley is just back-country tarmac, with the occasional footpath along field edges. Deeper in towards Broadhempston/Woodlad/Landscove you’ll find the occasional bidleway / greenlane, but nothing that’s especially worth going out of you way for. The A379 main road is ok when it’s quiet if you are in a hurry, but the top end can get a bit scary if you get drafted / buffetted by a big Riverford truck.