Joe it is indeed steep for growing. But the soil’s stones should reduce the risk of erosion if beds run up and down.
As you are growing for CSA and need to make money (different time-wise to veg for home consumption) I think horizontal beds with retaining sides along the bottom edge would take too long, being too expensive in time & also materials.
Tesi’s points are good for the Andes but in N Wales the relative lack of sun means stones warm less by day, and nights are relatively less cool. And they had cheap labour compared to us in the UK. While Sepp has many tree trunks he appears not to need for other uses.
All in all, tricky, keep us posted and I think you need, somehow, to concentrate on high value veg.