I'm thinking this is the cause of your bottoming out. Those springs are too soft. Sure, the ride is nice and cushy, but they let the rear travel too far over average bumps. Try ~5 psi in those bags. It shouldn't lift any more than about 1/2" or so. Drive for a few days around your normal areas and see if there's any reduction in the bumping.
If so, and if you really need that ~1/2" of drop back, you can just cut off one or two of those compressed coils on the spring. This will lower the rear the wire thickness of the coil while not changing the way that it rides. Those compressed coils are basically just acting like spacers. So, it'll be a little lower than it is now with no pressure in the bags. But, with a little pressure in them, it'll be back where it sits now and be closer to the factory spring rate. Really, this is exactly how the factory Auto-Leveling Control works. The rear springs in ALC-equipped trucks are a hair softer than those in the standard suspension. The air bags on the shocks are kept at around 5psi minimum at all times. This keeps the air bags in their proper shape as well as supplements the slightly "weaker" springs so that, collectively, the system has the same load capacity.
