Despite all adjustments that we've made with the hitch (in its lowest position) and the pinbox (in its highest position), we're still towing a little nose-high. The weight distribution of our 5er is ...
A subframe between the springs and trailer frame is needed to do what you are seeking to do. Full subframe with cross members, will keep the trailer frame from flexing side to side with additional torque of the springs mounted lower below the frame.