A burn isn't going to get the larger stuff, especially the stumps. You should have hired a bulldozer for a few hours and eliminated a lot of headaches that will now follow. I would swallow the losses on the bushhog time and get a bulldozer anyway. It won't cleanup as well as it would have before the stems were attached to the stumps, but a root rake will get most of them. You can spray the stems when they start sprouting out to kill the stuff, just go to the feed and seed and tell them what you are trying to get rid of. Dogwood and briars are great natural food sources for wildlife.