I think you should look into a versa wing...
My reasoning is that the more wing surface you have, the lower your wing loading is going to be regardless of what kind of weight you're carrying, also, at 965 mm span the versa is very close to your specifications.
The bloody wonder also has a pretty good wingspan to surface ratio and it should be easier to balance with the load on but you might have to scale it up a little since it's only 700mm if built to specs.