I very much doubt you are doing anything wrong. MItes happen, coccidia are everywhere and any time birds are moved or new birds are added they can be exposed to new strains. Fowl pox happens, especially when you live in a heavy mosquito climate. Mycoplasma also can be gotten many ways. When you first start with chickens it can feel really overwhelming at times, especially when it seems like everything bad that could happen has. And sometimes it does feel like bad things happen all together. And there are some things that happen (like reproductive problems) that we simply have no control over despite all efforts. It will get better as your experience and base of knowledge grows, we all have a learning curve when we start out. None of us knows everything, we just do the best we can and go looking for help and information when we need it. I don't use ACV in the water, despite so many saying it has benefits. With the summer heat and humidity of Florida I don't want anything to put them off water and risk dehydration or heat stroke, and my birds really didn't seem to like the ACV. So I stopped. Just something to consider. Your birds should be resistant to the strain of Fowl pox they had, so the ones that had it shouldn't get it again. You can reduce the incidence of that by putting window screen over all your coop openings. The birds are more likely to be bit at night when the mosquitos are more active and when the birds are stationary on the roosts. You can cut a piece of screen to go over your door attached only at the top, and cut in vertical strips that hang down, the birds can go in and out and it helps keep the mosquitos out. The screen on the windows does collect dirt and dust, so has to be cleaned off more often. Hang in there, it will get better.