lots of opinions already. I had something similar, old overgrown pastures I need to rehab asap when I moved in with my horses. We used a backhoe bucket to push over all the trees except a few older large diameter ones. Once in a while needed to use the backhoe itself to reach higher up and push, but most of the the stuff was 3 to 6 inches and the weight of the bucket was more than enough. Shallow root balls popped right up. I didn't burn them, rather pushed them up along the perimeter of the pastures as a brush fence since I needed to do fencing replacement once I had the pastures clear anyway, made a great temporary fix for the fencing. The soft wood was gone in a year and a half, cedar and hardwood I recycled into logs for fence gate posts and corners. The disturbed earth of the tree bases I just landscape raked and broadcast seeded into the exposed earth - because it was so overgrown it needed grass restoration anyway, so a good clover/timothy/orchard mix gave it a quick start. By the end of a couple of years the fencing was up, most the remaining wood on the ground was punky enough the bush hog "chipped" the remnants, and I have clear pastures with good grass (I kept overseeding the first couple of years, plus some ag lime). I liked this approach - could work it on my own pace with my own gear. I hate the pressure of a rental return, and given I needed to clear, seed and fence it worked out well.