Thanks! As to mathematics - count me in! I can't imagine these things in abstract form (equations) and need something visual. That's exactly why I made these spreadsheets. And once I had them I found them very enlightening.

The 18k ft case has been adjusted (Reynolds number, airfoil coefficients) and isn't quite as "quick" as before. Looks even better now, but still more drive efficiency is needed, that is less power per prop disk area. The airframe's power demand is a given so more prop disk area is needed.

Since I don't know of any bigger folding props I'd try the twin solution with the same prop and two smaller geared outrunner motors. Could be a true twin boom configuration with the inverted V-Tail split into halves (paddles). The two booms could be located quite far outboard for more aileron effect and a better mass distribution (wing bending load would be much smaller). The booms could be just tacked to the wing's flat bottom.