Old post, but dont always assume mareks, get fecals done, check for mites and start vitamins. My flock is positive and i vac any incoming since there is ancedotal evidence that it limits tumor grown. My deaths have dropped considerably. Check here for live testing...
Well I wouldn't recommend a metal shed, yes I've never had predators other than snakes, but oklahoma summers make it an oven. So you'd need a way to cool it.
I would definitely think nutritional. Do you have any nutradrench or poultry cell? 0.50 mg morning and night directly on the beaknor on a treat, see if it helps. I'd also send a fecal to the link I posted.
People jump to0 fast assuming mateks. A friend had a bird the vet thought mareks, euthanized, necropsy came back as a heavy worm load. No mareks. So I'd put that on a back burner for now.
Check near the vent for specks walking, that's mites. They can take a bird down fast. For worms send a fecal here
https://www.midamericaagresearch.net/