So sorry.Sad to say I lost 2 more. I am down from 10 to 5. One more suddenly looking ill this afternoon.
I looked at the dead ones. They are plump, perfect clean and neat feathers and feat / legs. No discharge from eyes or nostrils. Crop normal and no bad smell to breath. No spots or scabs, and inside the beak and down the throat are clear and normal. They also have good bright red combs (only my rooster has got pale come).
The only thing I could find is they had wet bottoms. Seemed to be pooping water. No poop smell to that area.. and feathers clean and not stained.. even on the white bird... just really wet and a bit matted. When I watched one poop it was mostly clear liquid.. and the consistency of water.. with a small amount of frothy light green in the middle.. which was also really flat and watery. No smell to the poops. So seem they stop eating but keep drinking.
I am now pretty sure it is not Marek's.... because of the symptoms.
I went to 2 feed supply shops, and a big vet supply store... which took me hours to get to them all... and NONE had ivermectin... just some other kind of antibiotic powder at one place. Really pathetic and annoying!!!!
I have just decided to let nature run it course. It's come to the point I am giving up keeping chickens because it's just too upsetting seeing my pets die.. not to mention the money I spent over the years on expensive show quality birds and supplies, coop, feed, medication, time to treat illnesses etc,
When I look at other flocks of chickens in the village they are just fed dry rice and either roost in trees or under baskets.. and left to fed for themselves every day wandering around the streets and yards. Yet my birds that I keep clean, good quality food, and dry predator proof shelter... they just keep getting different disease one after the other.

Wish there was something more I had to offer. I do understand your frustration about other chickens surviving despite fending for themselves. But I wonder if those are native chickens and yours are a special breed that don't have the same tolerances/immunity?
I have to be honest, I might throw in the towel too. But I'm still hoping for some survivors!

I don't look at all the time, $, and energy spent as a waste. I would have been doing something else anyways. But I would definitely consider the cost, time, and emotional toll going forward.
The ONE thing I can think of that is deadly and fast is maybe botulism..