I don't think anyone would say a drag radial is going to have less sidewall flex than a R compound tire for road racing. It is the nature of the beast.
If you want to throw non drag radial tires in the mix than look at a tire like the Michelin, Pilot Support Sports, Sport Cups, or Sport Cup 2. Those are great tires if you want a handling and soft compound. But make no mistake, no R compound tire designed to be home on a road course is going to hook like a drag radial. If you want straight line grip a drag radial will be hard to replace.
Maybe a R compound road race tire and Davis Traction Control System would work best? If you want to spend some Benjamin's.
If you are going down that route would look more toward the RE71R's, RS4, Dunlop Direzza ZIII. Usually they are cheaper than the michelin options and can offer a lot of grip before jumping up to an r compound. Still straightline not as strong as a DR.