I'm surprised with your outcome
@Glen
Thanks for sharing it in such a timely manner.
I understand what you are saying in that the bracket design is not up to the task. The camel's back is weak, there is more than one type of straw that can break it.
I'm posting this because I think that it is important for you to make sure that you do not have a harmonics-related issue.
After replacing my tires, and before replacing my fenders, my cracked fender didn't vibrate at highway speeds, so it looks like the radial tires solved the harmonics issue associated with the initial cracking on my trailer.
If your fender with the cracked bracket is vibrating on a smooth road surface, you may still have a harmonics-related issue.
I know that it's a pita, but if there's a smooth stretch of road that you can use for some test runs, it might be a good idea to use it to see if both fenders vibrate. If only one side vibrates, then switch the tires from one side to the other to see if the vibration follows the tires.
I'm only suggesting this because my fender with the cracked bracket did not vibrate with my new radial tires.