Going down from a 10x5 to a 9" diameter, it would be worthwhile to try a few 9"s of different pitch. The pitch we see on the props really is just the mfg's advertised best guess. Most people would suggest that a 9x6 would be the closest match moving down from a 10x5. But not many would bet big bucks every 9x6 would give the best performance.

Try a 9x7 or two.

And since you're using electricity, try plugging in a wattmeter to get some really good info. You can tell a lot about props from a wattmeter.