I've been slowly catching up to your circumstances as this thread has evolved.
So after reading the last post, I'm now editing this to state I think you've got litter/field management with crowding issues as the root cause. The illnesses are simply symptoms of that root. What size are your pens? Dirt will not allow for good litter control, nor I doubt the corn husks (though I've not used it admittedly....I've had terrible luck with straw which is similar...it just molds, doesn't absorb, and allows stuff to grow in it).
With 85 birds, you will always have something going on unless you are on top of field/litter and overall management. Chicken keeping is constant management. The larger the flock, the more occurrence statistically that can and will happen.
The larger the flock, the more poop, the more poop, the more pathogens build up. That's why field and/or litter rotation is essential.
You state you realize you don't have litter/field/pen control yet. Then, don't add more birds until you've got that resolved. My math suggests that 85 minus 15 still leaves about 70 birds to manage on ground that is not receiving appropriate treatment yet. You've just treated birds.
Until you get litter management under control, you will still be pushing pathogens into the litter and environment with crowding stress in the birds bringing on illness.
From the med list, I can deduce your vet is treating coccidosis, worms, and likely MG or a gram negative bacteria like Pasturella or eColi or Salmonella.
All of those illness are exacerbated by crowding stress and poop build up in poor litter choice. Get those things under control. You may be losing less simply because your numbers are now down enough to lower stressors. Also you've medicated but very likely not eradicated.
As to sand, I don't like it, nor do others in my area, because my soil is clay...like clay pot clay. I also live in a very rainy climate. Adding sand and water to clay creates concrete. (There actually is a famous concrete manufacturer in our area using our fine clay soil with sand).
Sand stagnates easily. I find it harder to change out. It is miserable to track.
However, there are those on BYC who thoroughly recommend sand. I think they are in much drier climate and have more loamy soil.
Just my experiences.
LofMc