That’s a nice cart. I was debating using lumber but Im gonna make one out of steel and on the bottom sides put a 6” high ledge one each side to assure boards never kick out . We load ours to the max as we do a lot of work in high rises and have to cart everything up freight elevators. Sometimes we have 20 sheets of plywood or heavyweight MDF and those suckers are heavy. The one I’m making is for our cabinet shop so I figure steel would be strong but less bulky than lumber but I really like your cart. Cute kid by the way.....little man already playing on the rolling jungle gym...haha.
The casters are probably the weak link, they are air-filled and not solid rubber. The cart weighs about 100 pounds, and I've put 420 pounds on it... It could probably hold more, maybe 600 or 700 pounds? Granite gets heavy in a hurry though! 😄