Most likely, your shoes are too small. When you first put on your shoes, if you can't wiggle your toes and splay them out a little, your shoes are too small and/or too narrow. Your foot is going to expand after running for a while, so a small shoe that's okay for an hour run might cause black toenail on 2-4 hour runs.
Black toenails are most often caused by wearing shoes that are too small, which causes the toe to jam against the front toe box covering with every step.
Less frequently, black toenails are caused by shoes that are too loose, which allows the foot to slide forward and jam the shoes with each step, especially when running downhill. I doubt if this is the case for you, but I'm throwing it out for your consideration.
You should consider the Hoka Bondi. Hoka makes some models that fit wide and others that fit narrow. I'm sure some of their other models have a wide toebox, but you would have to search online to find out. The Bondi will work for a bigger runner. The rocker sole of the Hokas will slightly reduce the tendency of the foot to slide forward because they slightly reduce the impact points of your stride.
It's possible that you have some form flaw that contributes to black toenail and broken toes, but small shoes is likely to be the culprit.