Imagine getting in and out of this. I don't want to climb through the window like a Nascar driver. That is why I use very heavy gauge tubing. It is 1.75" 0.120 wall DOM tube. This stuff is really stiff. Yes,it has a weight penalty but this is not a race car. I'm limiting the speed to 85 mph. I'm more interested in daily driver comfort and safety.
Also, the doors will be structural on this design, carrying some of the flexing forces.
I really appreciate the perspectives on this forum. It is very down-to-earth.