If you dug it out of your backyard, then it is a field grown tree and not a yamadori: it has not been stunted and shaped by the forces of nature on a mountain. Regardless, itseems like a fine specimen for a multi trunk. Do not cut anymore roots for now, pur it in a growbox, protect it from the wind and cold (maybe an unheated garage?) And let it grow unchecked the first year before cutting anything else. Good luck!