I had some of the same problems.
My track is minimal traction with black top soil.
First setup was the Japan all around setup that had no rear traction and was very hard to drive until I just changed to the narrow hangers. That really locked in the rear but took away a lot of the steering. What was actually happening with the wide hangers the rear end would give way and cause the rear end spin around. Just a tad to much power and the car would hook around. It was very unpredictable. Trying to steer with the rear end was very difficult but I could do it in practice but in race when I was trying to go fast proved to inconsistent.
So I switched to the Japan setup with narrow hangs and the car was a totally different animal. Good mix of side bite and forward bit but the car would push horribly. I let more experienced drivers of Losi and AE take it for a test drive and they all liked it but their comments where that it wouldn't steer. I even went to the extremes of the lowest roll center I could get in the front and the highest roll center I could get in the rear with narrow hangers.
Low roll center front = oversteer.
High roll center rear = oversteer.
but I still couldn't get it to steer.
So this lead me a quest to learn about chassis physics. I was going to marry the 2 builds and come away with a more balanced car.
My current build is my own and I think i'm on to something. I went back to wide rear hangers, shimmed the rear arms forward 2mm and the rear hubs forward 3mm(short wheel base). Rear camber link outer hole shock tower side 2m, inner hole hub side 0 shims. This basically counter balances the wide hangers that give the rear a much higher roll center then just adjusting cambers links. This still lets the rear get loose but not nearly as bad and in someways still feels like the narrow hanger but with steering. Even to the point it feels twitchy. So now i've got a car that has steering and rear traction. Still going to play with kickup and caster to calm to car a bit more. But if you know what changing something on this car does you can make it behave anyway you want.