Not quite, Cap! We do a lot of steel fabrication down here in the 'oil patch'. And 'steelwise' we are a lot more generous than you are. Since your are very good at welding; I would add 1/4" thick 'brackets' shown in red (yours are on the 'skinny' side). This would beef up the X axis section modulus and should take care of vertical forces. I am still worried about the Y axis section modulus and horizontal bending moments. Go easy backing up or stepping on the brakes.
It seems like a steel plate a couple of inches wider than the tube welded onto the top of the tube would eliminate horizontal bending moment. To reduce weight it could even be designed in a pair of triangles. Just something to make the center of that tube stiff.
Tows really well, no issues put about 200kms on logging and forestry roads this week, Moose, Mountain Goats, Mountain sheep Even a few Grouse joined me for dinner one night.