I'm probably the extreme case of someone with lousy back who still kites. My solution is pretty Harsh but it allows me to keep kiting. Had low back surgery when I was 27 (around 1986 and missing about 30% of my l3-l4 disc ever since) and have always had to use a seat harness for kiting. Started with the Fusions but I found that the key to reduced back loading was keeping the spreader bar locked way down low at the hips, basically at the l4-l5 elevation or even a bit lower. All that extra back padding of the Fusion was worthless if I couldn't keep the bar low.
Think about it, if your seriously powered up, and edging big time while sending the kite low in the power zone, all the harness lumbar back padding in the world isn't gonna prevent the load from transferring to your spine if the bar is high. With the bar low enough all the load is transferred from my hips right to my legs. My whole upper torso above the hips is just along for the ride. With the kite high in the window it's not an issue, but who rides with a kite parked at 11?
I found that if I took that very bottom d clamp strap on the bottom of the Fusion and sewed even tigher than it's max closed position, I could force the bar lower and keep it locked lower without having ride up during the session. The bottom of the harness acts more like a real seat, but if your upright when you kite (most of us are!) the bottom of the harness digs under your butt. It's not the greatest feeling (have session red marks to prove it) however it locks the bar low and allows me play instead of giving up.
Anyway, I graduated from the Fusion and did the same thing with a pair of Nitrous shorts. Even had a boat cover shop do the stitching I specified. Riding that way ever since. When I foil I can even loosen the straps up a bit since the loading is so much lower