I see loaded tires as a complement to a heavy rear implement for loader use, not a replacement. When you have a load in the bucket, you have effectively created a see-saw with the front axle as the fulcrum. Loading the rear tires keeps the back end on the ground, but presses the front axle down with twice the load that is required to hold down the back.
If you put a heavy load on the 3 point, you drastically reduce the load on the front axle, because you're moving the fulcrum toward the rear tires, rather than the front. This reduces front axle stress, allows for much easier steering, and gives you much better traction. Even with 4wd, minimizing front axle strain is only better for the tractor.
Since you also want to change the tires out, I would suggest going with something heavy on the back end of the tractor. If you just have a scraper blade rather than a box blade, you could maybe devise a bracket that bolts onto the scraper frame that holds
ballast. I have seen the concrete boxes before, and understand the shorter overall length being attractive, but I'd rather have another tool out back, as opposed to an otherwise useless lump. I have suitcase weights that I hang off my scraper blade. It makes the blade work better, and gives me the ballast I need, which seems like a win-win to me.
It's pretty obvious what my bias is, I think. I'm defaulting on the side of reducing stress on the tractor and making it easier to drive. There are many threads on building weight boxes; if you're going to do that make sure to put an area to carry chains or straps, and some PVC pipe to hold shovels, rakes, or other hand implements, and maybe a chainsaw. Putting a receiver hitch, or at least an attachment point for a towstrap seems like a good plan too.
I'm pretty strong, but there is no way I would (or could, really) handle rear turf tires that are loaded to change on and off without a significant mechanical aid. They are just too heavy, and once they start to tip over, one guy cannot catch them. If you're going to do it, an engine lift or something equivalent seems like a necessity to me. One problem is maneuvering the tire around and keeping it upright, too. I would want a rack of some sort to keep the tires upright in storage. They are too dangerously heavy to be tipping over onto boards or something, in my view.
One tip, if you're going to change them out yourself, would be to put a wheel stud into one bolt hole on each hub. Having a stud gives you a target to "catch" the rim with and then work around to align things properly. Even if the height isn't quite perfect, you can get the stud into another bolt hole on the rim, then manipulate the jack or tire to get the remainder of the holes aligned. One per side is all that's necessary.
If you're worried about traction and need more weight on the tires than your ballast box/scraper allows, I would put on say 4 studs per hub, then build a spool with a short piece of weightlifting bar welded to a plate with a matching bolt pattern. After your wheel is mounted, the extra length of the studs would fit through the plate on your spool, then fastened with nuts. Install weights in whatever manageable increments (probably 35 or 45lbs at a time) until you're happy, then lock it on with two of whatever retention system the weight bar came with. You should be able to safely add an extra 135 lbs or more per tire that way, and it would be simple and safe for one guy to do by himself.