I dont think its gonna flex anyhow. It will simply pivot on the coupler. I had a similar suspension on my old trailer, which was/is great for washboards and potholes, but it just pivots on the coupler, as its only being dragged behind the vehicle, and the axle isnt powered to initiate any torsional stresses. Sure, YOU can make it flex, but when its being dragged behind a vehicle, its not going to flex like a crawler would. It just pivots.
For example, here is what will happen. Notice the axle is still parallel to the frame? Its pivoting on the hitch, yours will do the same, even with the single suspension pivot point in the middle...because of where the bags are mounted. It'll work great as a supsension design, do get me wrong, but it wont flex like you are thinking.
Looks great so far BTW!