So I was watching the video of the last run on Saturday. So it hits the tire and then tries to throw the axle out of the car. At the hit it plants the tire moves forward about 6” then the body separates until it tops out the rear travel and unloads the tire making the tire spin. the tire then bounces with the rear suspension at max lift all the way through the 60’. I think you are pretty close to full stiffness on rebound. Either you continue going stiffer on rebound to slow the separation down or you could lower your percentage of rise number. Basically drawn a line from the center of the contact patch of the tire through the front spring eye. The farther forward that line is angle softens the of the tire slowing down they body separation. .