Man, you guys are building tanks. My trailer weighs 1,200 pounds. The tongue extends 48". The tongue weight is 100#.... the tongue is 2x2x5/32" The only reason I would put any diagonals on is to mount a tongue box, and they would be 1.5x1.5 angle. There are two braces (see pic) on each of the two attachment points. I have over 15k miles on it with no problem.
The loads are the tongue are primarily vertical, not horizontal. I'm thinking about making my next tongue out of laid up pieces of 1/2" overlapped plywood.... Yes, the trailer bounces.. up and down. side loads are minimal. Even bouncing up and down, the suspension and wheels take most of the load.
The box taped onto the tongue is the wireless sending unit for an inside/outside thermometer.... don't leave home without it.