That's all accounted for. There are just no big enough folding props. And I think fine-pitch is a makeshift if your drive is actually turning too fast. At least the prop can be big enough for good efficiency, but instead of gearing the drive rpm down the prop pitch is reduced. Peak efficiency is generally achieved at advance ratio of about 1, however well the prop is designed and made. I don't know any 3D model, heli or even parkflyer that has an efficient prop. Would be just too big and slow. Even full-size props on smaller airplanes are not efficient because they would be too big, low-rpm engines would be too heavy, and a gear would be too delicate and expensive.

By the way, I have a restraint writing your RCU name because this name does mean something to me. Are you the inventor of the jet engine or is it your real name?