Great build on the trailer, I like it. Suggestion on the tongue jack. The top bolts in as you did but there is suppose to be a flat plate on the bottom of the A-frame that has a round hole in it for the jack to go through, causing it to stand firm and not lean like it is doing in one of your pics. Don't weld the jack to the bottom plate, can easily be slid back out if ever a problem. Take a piece of plate or wide flat bar to fab up a bottom plate. That jack was designed to withstand a lot more than you will ever put on it with your small 1 axle trailer.
Also, to keep from doing a lot of twisting when using the jack I drill one hole in the bottom of the leg of the jack and then slide a piece of pipe INSIDE of the existing leg that has holes drilled every couple of inches so I can pin the inside pipe where it extends the outside one. I weld a piece of flat bar or plate to the inside pipe to form a foot for the jack. When unhooking, I pull the pin, drop the inside pipe with foot to the desired position, pin it off and start twisting the jack to raise it to the desired level. Saves a whole lot of jack cranking and the inside pipe never gets in the way, fits in there perfect, no problems. I still drop a piece of board or wooden block for footing if the ground isn't really hard.
I like your welder cart, got 4 wagons I picked up at auctions that are used for shop/garden/yard projects. Works like a dream. I have one rigged for fishing, rod holders mounted on plywood that mounts or dismounts with 4 sheet metal screws. I pier fish some (ocean) and go to one lake where I like to walk the dam, no car access. Don't always take the boat. Lots of uses for that wagon.