Added trailer hitch, strong enough to pull ATV trailer?

You certainly are no novice at welding. Good job there. I would think that as much flexing the frame of a TT does you should keep an eye on the welds themselves for cracking either in the weld or in the frame. The frame is not likely to be 1/4 in steel so to me that is the weak link. The welds at the bumper may crack also even though they are not there for strength, there will still be a load on them. Flexing and work hardening is the enemy of welds.