What Roadrunner said!
I like your collar spacer idea.
A point of contention though. I regard the Trax2 cart frame as being the toughest, most reliable cart as it has a wider, more stable solid axle as opposed to a hollow axle on the new HD cart. Also the clips on the new HD cart are easily dislodged and lost.
The Trax 2 cart is cheaper and the money you save can be put towards adding the HD wheels.
Fitted with the heavy duty wheels as detailed above you have the choice to easily swap back to the balloon wheels depending on the terrain.
IMO it is the best cart option!