Go-oood Luck
I don't know of any way to get all the thorns picked up after felling - nothing short of a whole lot of manual labor is likely to solve this problem. Although, if you could drop one tree on top of another in such a way that only the branches of the first tree hit the ground, the job would be much easier.
I have used blocking on the ground to protect water pipes when trees could only be dropped in a particular spot and noticed that if the trunk hit the blocking before it (the tree) canopied (rather than the canopy hitting the blocking), the tree was likely not to break up when it hit the ground because that section of the trunk took the hit.
We have black locust here - smaller thorns, but a similar problem. I spend a couple of days in the Spring picking up downed branches (big & small) from my 1/2 acre. Locust is great firewood.