Its cool to see all the different ideas. If the majority of your towing will be at highway speeds, then, as others have already said, aerodynamics will have a much greater impact than anything else. Since everything consists of engineering compromises, make reducing drag a priority over reducing weight.
The security of four tires is nice and the dual wheel setup appears to be a good alternative to a tandem axle setup. Anybody have thoughts why dualys wouldn’t be a good compromise?
Springs or no springs? With no springs the unsprung weight will be huge. Hit a large bump and without springs to absorb the impact energy you will see much higher peak loads and stresses to the trailer structure. Remember you will have to add the trail rig’s unsprung weight (tires, rims, axles, and parts of the suspension) to the trailer weight to get the total unsprung weight on the trailer’s axle. You can go without springs, but especially if your planning build the trailer as light as possible, you may see fatigue issues over time.
Try to design for stability. Every trailer will sway at some speed, even properly loaded ones. Well designed trailers will sway at speeds higher than you will ever reach. Making a compact trailer (short) will make it more squirrely than a longer one. Think of a grandfather clock pendulum. The longer the pendulum is the slower it sways side to side. The shorter it is the faster it will sway. Also remember the tow rig will enter into the stability equation.
Looks like your tow rig has ok wheelbase, it is relatively heavy, and has stiff sidewall tires (if they are the stock ones anyway), but that long rear overhang is going to work against you. It will give the trailer more leverage to push the back end of the van around during cornering or during sway conditions. As others have suggested, a weight distributing hitch with sway control would probably be a good idea.