I'll just try both! That's the beauty of experimental motors. I'll first try the sugar rockets as James' tests looked promising. If that doesn't work, I'll try dabbling with something else. As I mentioned before, though, I am more of an RC enthusiast and not so much a rocket enthusiast, so I probably won't do too much experimenting.

Also, do you think it would help if I increased the diameter of the motors? This would cause more propellant to burn and might increase the thrust a bit.

The other idea is to drill into the propellant a little, creating an initial core-burner for a powerful takeoff, then a long end-burner would follow to sustain the flight and add altitude.