Did you ever need to upgrade the tires/tubes to hold 400 lbs??
Not the tires and tubes, no. Just overinflated them a little. I was concerned about the rims and spokes, but they seemed like they were reasonably beefy. I didn't plan on doing any jumps or stunts with the trailer, so I was reasonably confident at low speeds.
If I were to do it again, I would have started with a trailer with forked wheel mounts. On mine, the axle inserted into a quick release bracket from the side. I think yours do the same thing. They flexed a little more than I liked; they would have been the weak point if I loaded mine any heavier. Something like this would be inherently stronger:
The axle is attached on both sides of the wheel, just like the axles on your bike. A little more difficult to install and remove, but not too bad.