The collapse is caused by the profile being too positively/high cambered, like your hand when it is cupped. This can be either caused by the bridles or how the kite is sewn/condition of the fabric. To fix it you need to make it more negatively/reduced/reflexed cambered, think flexing or pulling your hand back. You can do this by either shortening B and/or C, or lengthening Z and or C!. The simplest solution is to just lengthen Z as you already have the adjustment. One thing you have noticed, heavier bar pressure, is a symptom of Z being shorter/tighter. This is because of the pulley setup, all force on Z goes directly to the bar. Z being tighter also makes the kite higher camber which also moves the center of pressure further back while increasing it too, so generating more force further back , i.e. more on C and Z etc, i.e. heavier bar. The reason higher camber causes collapses is because it generates a moment of torque that causes the wing to flip forward. Think the rear of the wing deflecting air down, so newtons law, it wants to do the opposite. A negative camber will do the opposite, wanting to flip backwards ,but because of the towpoint or bridle the kite can't do anything really, except be more stable and perhaps lose some efficiency. You can go into a myriad of ways to alter the kites camber from playing with individual bridles to sewing pleats on the canopy. Best to play with mixer first as it is simple and easy, as I said lengthen Z. You could also look at restretching your bridles. One other thing is the final tension B has when you depower can affect the stability of the kite depowered, you can play with that by either just adjusting B, but that will also change the camber/profile or you can adjust all of BCZ or change the length or put a stopper in the pulley line. Moving that knot attaching Z would eventually affect this. But yes complicated
, just adjust Z and your problem should be fixed.