No additive? damn you are gonna have problems.
Mulsanne has too much rear grip, Corvette feels more balanced, but if you are on untreated surfaces then you are never going to find a happy balance.
I can only suggest very soft rear foams, like Corally Silver Stars, but they can chunk easy and wear out fast.
Try removing all front droop, I run with no droop as it gives too much front bite on our high traction purpose built outdoor track.
and is that a 5.5BL motor you run?
way too much power for a low grip surface