If you think it'll need 4 to 4.5 degrees camber to get front tire temps correct, it really does sound like you're just rolling too much. It makes sense, you've got softer springs; so increasing front bar stiffness seems like the logical next step. If it then understeers too much, then it might be worth trying a rear bar to get the balance right.
I understand the reluctance to add the rear bar, but even really big horsepower cars in the pre-ground effect era usually had rear bars. I think it's useful to look at professional road racing cars from the '60's and early 70's for ideas about our cars; the level of chassis development and downforce is very similar. For what it's worth, Carroll Smith wrote in Tune to Win that he generally ran the softest springs he could, and achieved the roll stiffness and balance he needed with front and rear bars.
At any rate, I'm enjoying watching your progress!