This is all because because I am looking at this in a different way. After towing a race car many 10's of kmiles to Canada and all over the eastern and central US, I think in terms of what happens if I wreck or someone pulls out in from of me, not just what will hold the car on in normal use. I would not like that car coming through the back window....
So you could pick up a 4000 lbs trailer with one ring....so what? In an impact, the G's will make your car weigh 15000-30000 for a fraction of a second. If it is held back by just 2 rings, you can see how they will fail. As I indicated, OK for holding the car on, but not adequate for a wreck. That is why I suggest a direct tie to the frame with chains on the rear of the car. All those webs hold-downs and D-rings have a very real limit in a wreck. But I realize that folks sometimes don't worry about this factor so much as I do.
If you look at the tongue area from an engineering viewpoint, you'll see that about 80-90% of the tongue weight is supported through the center tube; the side diagonals are small and oriented with the wide dimension flat so they will not take any significant vertical tongue load; they only stabilize the tongue sideways. So about 80-90% of the tongue load is going to go through the center tube alone; that is just the way the layout you have works. So vertical gussets on the side tubes won't add much.
Again, you have to look at the dynamic situation if you hit a big bump, and the tongue load increases 5 or 10 fold for a moment. If the center tube is thick walled, then it may be fine. If thin walled, it just may buckle; we don't know the wall thickness of that center tube. And I was just giving you a comparison for a modern trailer with two 6"x 2" boxes oriented vertically, one on each side.
Budgets are understood... when young, I got a similar light trailer.... Jesus, it had no springs, a single axle solidly welded to the frame, and danged 3 bolt Renault wheels with no brakes!!! LOL It towed two cars total with me worrying the whole way before I scrounged and put in 2 5000# axles and beefed up the tongue area....
