I agree the germination rate is low. That is why I collect the seedlings from under the tree in my yard. I am guessing the germination rate is well under 1% in that situation.
I did a mix of partial defoliation methods on the trifoliate leaves. Sometimes I cut out the largest leaflet and cut the others back a bit. Other times I cut all three back by about 2/3. I have also done this will a box elder (Acer negundo) seedling that I have been growing in a pot for a decade because it had a nice textured bark. Still a very coarse tree, but smaller leaves and internodes than the one in the yard.