OK, I'll give different props a try. That makes sense, as during the takeoff run it's not going very fast so the engine is nowhere near the RPM where it puts out peak power. Bigger/skinner wheels did cross my mind - full scale Cubs, etc. use tundra tires (very big tires) for operating off of rough terrain.

Maybe a combination of better prop and bigger wheels will improve things enough... Also, I suppose I could try JATO (kidding).