Different strokes for different folks. I myself like lots of power, my old FLM (hybrid bulks) Maxx was a Castle 1520 on 6s. But my throttle finger has plenty of training with high power setups in 1/8 buggies and the lower 1600kv helps tame it a bit too. 6s with a 2200kv is a lot snappier than the lower kv. I personally like 5s for a 2200kv for that extra pep without getting too nutty.

Your Trenchers aren't really the cause of the truck getting uncontrollable at 40+mph. Pretty much any MT tire at those speeds is ballooning which makes handling kinda squirrelly. 6s is a lot of power and needs thicker diff fluid. On the throttle with 6s, tires ballooning causing less traction, a diff unloading at this point will not help you control the truck.

I know you said you were changing diff fluids but I can't remember what weight you went with. I'd try 50k front and rear though. Taping the inside of the tires to prevent ballooning also helps. I run taped 40 series (6.89" tall) Trenchers on my LST XXL geared for over 50mph and it's nice and stable. Haven't even gotten around to balancing them yet either.