The Pine Trees should have a pretty shallow root system so you probably have most of them.
Several years ago a neighbour got discouraged with my rotatilling and the results I was getting with my 16
Hp. Kubota. He set me his 65?
Hp. Case Tractor with a 7/8?? ft. Rotatiller. It went down about 13 inches the first pass. More passes and it was deeper.
This tiller was sturdy. It did hit a few rocks and did break a few shear pins but no damage to tines or even thinking it was struggling. If I recall the tractor was in creeper gear and at about 750
rpm. In my opinion it would have handled some pine roots with no problem other than winding them up.
A heavy duty tiller sized to your tractor or maybe just a little less width should work.
Consider making a drag for leveling. The width, angling and going different directions will do wonders. Eddie Walker has a thread on one he built that looks pretty good.
Those are my thoughts.
I'm sure a heavy Rome type Disk behind a D6 or a proper one bottom brush breaking plow behind a D6 would do the best job. These were the type of implements I have seen used to break newly cleared brush land. Quite a few of the disc's were behind a D9. But this type of equipment is probably no longer available.
