Ya, I wouldn't use that trailer to haul garden seed around my property pulling it with an ATV. If you have a Cargo Pro dealer in your area they make a great raft/cataraft trailer. Rear roller, winch, tilt deck, and all aluminum. Great deck material too unless you get the cheaper one with 2x6 wood as a deck, but still nice. Might set ya back $2K. But this boating hobby costs money.
As mentioned, a 3500lb axle is a standard item on a real trailer. And 15" wheels are the way to go. I think mine are 14" wheels at the moment and they work fine. Avoid the "doughnut wheels".