A few more thoughts:
The low point of the trailer, of course, is the axle, so Thomas' idea for the Timbren axle-less axle might be a good start.
Beyond that, the levelling jacks are pretty vulnerable. On two different occasions, we have caught one and bent it badly enough that we had to replace it.
The rear end will drag now and again. But because the trailer's wheels are set so far back, it drags less than I expected. And of course you have skids welded to the frame. Would it be worthwhile to weld on a full width skid plate?
In my experience, the front end of the trailer - the hitch area - drags more than anything else, as the tow vehicle's rear wheels drop into swales and depressions.
For example, on a back road in Oklahoma, we ripped the propane carrier off the frame of our old TM. Fortunately, we were near a farm whose owner had a welder in the barn and was willing to reattach it. He offered to do it for no charge - nice guy! - so I thanked him and slipped a weighted twenty onto this welding cart. It was worth a lot more than that to me.
The jackstem has always seemed particularly vulnerable to me, though we've never caught it.
For a long time, we dragged the shank of our WDH quite often. I finally had a welder cut 4 inches off the bottom, but it still drags on occasion.
Of course the springbars will drag, but if you are moving slowly you can always stop and remove them until the obstacle is passed. Since the springbars are oriented fore-and-aft, and high at the front, they are almost like skid plates themselves, though not nearly as strong.
I don't know if it would be practical to add a skid plate up at the front of the A-frame, but I'm quite sure no one else here on the forum has done it. You can be first!
Bill