Very classic with taking cross out at you cross number and left number. You moved cross closer to left. That always tightens a kart. Same with say 50 cross and 55 left, raising cross tightens the kart. Cross cannot be used to force a kart to handle. Period!
You mentioned your seat being centered between the steering shaft. Where is the back of the seat relative to the brake router?
What is your height, weight and where do you carry your weight?
To me it seems like something is very stiff at the RF causing too much weight transfer at the RF.
I did talk to a well known pro who is a good friend of mine about the kart as well and he told me the kart is just to softly built. Fwiw.