A question for you - aren't those suspension arms on backwards? I expect to see the suspension arms fitted on the same side of the axle
tube as the long side of the mounting brackets - just like on your old axle
As it stands in the new axle photo, the weight
of the trailer is being carried outside the mounting brackets, putting extra strain on them, plus the bolt at the opposite end will be permanently in tension as the mounting bracket tries to pull away from the frame.
I'm not saying I know it's wrong - just that it would be worth checking if it's right.