My flock has MD and I lost a few who declined very quickly, before we knew that had it and another few when we were figuring things out. But I have found that good feed, reduced stress, minimal treats (outside of healthy ones) and lots of room has helped keep my flock healthy-ish. We do still have GI flare ups which I treat with corid. I'm starting a monthly regimen of corid treatment to see if this can help but it's Marek's, nothing will cure it.
I drove myself crazy with research and spent money on "remedies" but the best way I have found is what I said already. I have found I manage it best with the necessities they need. I stay mindful of their health, help when I can and have compassion when I can't.
My flock also has MG so they are on monthly Denagard treatments. I am fortunate to have as many healthy chickens as I do with only the basic necessities they require.
I switched them all to a high protein crumble for the winter with access to oyster shell and dried crumbled egg shells. Molting is very hard on them. I did bring a naked girl in for the below 10 degree nights and do have a panel heater for the hard molts if they choose to be near supplemental heat, which 2 did. One is already back on the roost with the rest of the girls and the other is starting to move away from it. This is one of the "spoiling" treatments I do to try and minimize their stress. I also add Nutri drench every so often to their mash to make sure they are getting vitamins because of the random GI issues. This is our first winter so I will watch closely, do what I can and pray for luck

We did purchase 5 vaccinated chicks with hope they stay healthy. Losing our chickens has been really hard. My son has had a crash course in life, death, compassion and what it's like to be responsible for another life. I am really hoping he can get attached to these ones as he knows it's a matter of "when" and not "if" for the others. Good luck to you!