Did you consider installing brakes on both axles? I recently added brakes to the second axle of a 16' trailer I use for my tractor (not as nice a design as yours :c).
Sure makes stopping better and they are not expensive.
bumper
Most of the time I won't be carrying more than 3500 lbs and a single brake axle behind my 1 ton dually should be fine. If I find I want more brakes on the trailer I can add second set of brake drums. Second axle has the brake plates in place.
The trailer design came a company in Australia. Had to change the A Frame. They use a 32 degree angle instead of 50.