I am so terribly sorry you are going through this and I completely understand what you are suffering right now.
Hindsight is always 20/20 and trust me, everyone who is in your shoes has thought and felt the same way.
For me it was, why didn't I buy vaccinated chicks instead of breeder hatched babies. I'm still kicking myself for that one.
The thing is, Marek's is EVERYWHERE. It's like cold sore virus in people. Just about everyone is carrying the cold sore virus in their systems. Some have resistance to the virus (which is a variation of the same herpes virus that causes Marek's Disease in chickens) while every time other poor individuals go through a stressful period in their lives they get a huge cold sore on their lip
So it goes with chickens.
Why are some breeds of chickens immune to MD and others aren't? Look at Egyptian Fayoumis. I have 5 in my flock that are genetically resistant to MD plus are vaccinated (3 hens vaccinated one hatched/second generation). The oldest are three two years old now and the youngest 1 year old and they so far, knock on wood and cross my fingers, are doing great. I love watching these long necked, fiercely independent birds scratch around their big run in search of bugs. I pray that the scientists doing research into what makes these birds genetically resistant to MD so it can be passed on to the other breeds of chickens that we have in our flocks.
Till then we can only do what we are doing and that is hope that some of our chickens survive this horrible disease and build up resistance to it.
Silkies are one of the highest susceptible birds when it comes to MD. The birds that I lost the most to MD (and yes, at a rate of one a week and sometimes 2 a week) were Buff Orpingtons and yes, I felt crushing defeat when I read that they ranked up there with Silkies when it comes to MD.
Just hang on. Is my best advice. Your losses are horrible and I feel your grief and helplessness because I have been where you are. But it will get better and it will end. You will lose a lot of birds but you won't lose all of them. Then you can develop a plan. Mine was to bring in local resistant birds and vaccinated birds. So far I am into year two of not losing any birds to Marek's disease. The year isn't over yet though so I'm still keeping my fingers crossed.
I wish there were little t'shirts for birds that said 'I survived Marek's Disease'. The funny thing is, if you can call it that is that dealing with the Covid pandemic has been duck soup for me. My flock has been dealing with their own pandemic for 6 years now. If they can do it, so can I.