I guess the 60% throttle required occurs in both forward and reverse directions? Or do the motors only spin in one direction? Our current skid-steering support really expects motors that can spin in both directions.
My initial thought is that it would be best to get better performing motors and/or motor drivers because any changes we do in software will just be masking an underlying hardware issue. In general it’s best to get better hardware than the try and fix it in software.
It may be possible to get rid of some of the deadzone by increasing the THR_MIN value. It might also be good to provide a log showing the problems.