I use a short stick (less then 2 feet) with a diameter of an inch (more or less)
I rub it fast up and down along the shaft before cutting it. I dont put pressure into the shaft, just slide fast up and down all around the shaft.
That usually detach almost all the thorns and make the cutting esear.
Even if some thorn stay in place they are usually blunted and the shafts are good to be dried as they are going to be sanded after anyway
You may have a very small amount to be done with the knife but it is usually a fast process
I do the same when collecting brambles to be woven into baskets
Another technique I dont use often (for laziness mainly) is to open a hole the approx size of your shafts diameter into a tin can and run the shafts through it