I watched your car a fair bit as you were on before my friend. From what you say Id try taking out that rear weight so far back. Although its pushing dowwards it will also want to swing sideways.
Moving the weight foreward should give understeer a bit more on power and make it safer (in my opinion)
Something else id try would be adding an extra degree of rear toe and use a rear anti roll bar. The extra toe should lock rear in while the roll bar should take a little grip away but also make it break away in a controlled way as opposed to gripping loads then suddenly rotating like it was.
I add weight to the rear of mine when I want the rear to break traction and pivot easier
