The takeoffs in your video are exactly what I was talking about - gun and jerk. It almost cost you the plane in the second takeoff when it stalled about 8 feet off the ground. Look at how the big commercial jets and military bombers take off to see how a heavily loaded plane should take to the air. It does require more pilot skill, but you'll be amazed at how much more weight you can carry when you learn to takeoff properly. It will also allow you to design a plane that flies at a lower airspeed, which lets you use a bigger prop to overcome the drag that all that extra weight is going to create.
I'll make one other suggestion. I see lots of planes with a boom fuselage like yours, but there isn't any good reason to do it that way. It probably creates more drag than a smooth shape would, and you can probably save some weight with a balsa box back there that has some width to it.