It sounds like you're at that magical point where the wing just won't give up flying. Much of lift comes from the top of the wing, and that has to "go away" for a good harrier. That's why some planes with a load factor that's a bit high can do a harrier. The wing needs to act more like a boat hull on water. Maybe the extra 10 degrees you found will do it, but you may creep the CG back a bit at a time also. Also, try it inverted. The height of the Su-Do can cause the CG (in the vertical) to be a bit above the centerline. This can cause a "not so good" harrier upright, but a good one inverted.