I'm so sorry about your cockerel. There is a good article here, my journey was similar:
https://www.backyardchickens.com/ar...-i-learned-to-deal-with-mareks-disease.76944/
I also have Marek's in my flock, I decided to try to breed for resistance. That is a years long endeavor, and I'm a long time from any conclusions on that. So far it's manageable for me, I could change my mind at any time. There are many articles on Marek's. My personal experience is that symptoms can vary greatly depending on the strain you are dealing with, and each birds immune system. I've not had the progressive paralysis that so many articles talk about, at least not yet (going on 3 years). I've had mostly occular symptoms, with some with visceral tumors. Some have taken years to progress, some have shown no symptoms until they get ill and pass within a few days. Those are the ones that I find tumors in. I've lost young birds and old. I still have birds in the 7 - 8 year old range that are still symptom free.
What I'm trying to say, is that the literature is great for learning, but your personal experience may diverge from that, at least sometimes. One of the worst parts of Marek's is that it can compromise the immune system leaving them open to other things.
But it can be manageable, it just really depends on your own philosophy on keeping them and what you are willing to deal with. Sorry you are dealing with it, and I wish you the best, whatever you decide going forward.