This isn’t a dealbreaker for me. In agriculture, I’ve observed that many things decompose in the soil, particularly when they’ve been burned. Nothing endures forever, and if nothing prevents you from continuing to burn it, perhaps you could treat that section as a trash and burn area?until you feel it has sufficiently shrunk and broken down. Meanwhile, I wonder if you might consider looking for a better property, especially if you have cash flow and this issue truly concerns you. It also seems possible to cover any remaining fragments with a substantial layer of dirt just let it settle naturally.