WoW, 9000 ft ASL. You definitely need the bigger engine, and I'd try an inch more pitch than the manufacturer recommends, to start with.



Keep in mind, tho, pitch determines the angle of attack of the propeller blades, and too much is likely to be worse than too little. In the interest of simplicity, the formula I proposed above to determine optimum pitch does not take air density into account. The slippage I described would be greater at high altitude, since your jello is pretty thin up there. ; p



Theoretically, less pitch and greater diameter would be better for hovering, but as you noted large-scale models using powerful gasoline engines and relatively slow-turning propellers use high pitch props. The reason they do this is because they can! With an abundance of power, they can turn those props effectively no matter what the airspeed is, even zero! Note that pattern airplanes also use high-pitch props, although most of the maneuvers have vertical components.



Some full-scale aircraft have adjustable-pitch propellers, so the pitch can be adjusted to best suit the speed of the aircraft, and the speed of the engine(s). It's only a matter of time before this technology will be available for larger models.



In the meantime, noname, I would go for the pitch!