Fantastic build. I would normally not think a lot of wood walled trailer for off road, but with the effort you have put into this, and that logging trailer frame you have built, you should not have any problems. Most skimp on the frame and it destroys the trailer down the road. I used to build 20 foot car haulers with 2X4 box, most still on the road. Over kill is good, it is your friend. Only concern would be bouncing as the frame is so rigid. Be sure you go light on the axle, the trailer can not weigh over 750 pounds is my guess, and most of that is frame. If you over spring it it will bounce, just remove the leafs until your happy is my suggestion, or go with a torsion axle down the road and set the weight low.
Again, having built many off road trailers back in the day, wood was normally a key to failure, this one is not. Great job man.