Those two pictures show pretty clean property. I don't want to discourage your obtaining a good grapple. They are a real God-sent device as far as a time saver and back saver.
You might want to consider going between the rows of planted trees and mowing down the weeds.
I don't think I have any quite that big. Seems like they cleaned it up half way decent. Should look good in about 5-6 years when the pines start to take off.If that picture on the right is one of the piles ..... I would not bother with them. They will rot down faster than you think. When I think of a pile of slash - 30 feet in diameter - 15 feet high .....like the ones I had here on my property.
I'd chop those weeds down and then plant some type of local cover crop. That would keep everything in its place - curb erosion - until you can develope a final plan.