I'm not sure where you live, but I had similar symptoms in my birds, so brought several for necropsy at the University of Missouri. I live in central Missouri. I was having huge losses for no apparent reason when I had never done so before, and certain colors were dying off at a higher rate. The results of the necropsy showed Lymphoid Leukosis. I have purchased birds and eggs from a number of breeders in and around the area, and it can be passed on through the egg, so you are not safe only purchasing hatching eggs from breeders. While I suspect who I got it from, there is no proof unless they also tested their birds (which they have not done despite being notified of my results).
I also have silkies, and seramas. The second generation of the seramas seems to be less susceptible to it, as only the birds that are less prone to develop the tumors lived to reproduce. I am now on my second generation of silkies, and they too also seem more resilient, but certain colors/strains of my birds seem more susceptible to infection and developing tumors. Also, my birds (as their immune systems are compromised by the infection) are more likely to get extremely ill from other infections which would not normally cause issues in healthy birds (such as coccidia). Because of this, my birds are started on a moderate dosage of Corid as soon as they are around a month old (or showing symptoms) for 14 days (as recommended by my vet), and are given vitamin supplements several times a week the rest of the time. I keep them nice and warm, and keep their stress levels low. This has reduced my death rate from around 50% to approximately 2 to 5%.
My flock is now a closed flock. I do occasionally introduce new blood of high quality birds to keep my flock from inbreeding (and because I love the breeds and refuse to destroy my pets simply to start another flock). I practice very strict biosecurity, and I only give away birds to those individuals who are fully aware of what my birds are carrying and my extra roos now have their own space in the garden, and earn their keep by eating bugs off the plants in that large fenced in area. I have notified everyone that has ever gotten birds from me. My vet, who is an expert in his field, has noted that this is actually a disease that is becoming quite prevalent in the backyard hobby because: many birds survive the initial infection and are carriers of the virus which they can later spread to other birds, and it can be spread from hen to chick through the egg. I now breed for those silkies and seramas that are hardy and more immune to developing tumors, as there is no vaccine or cure for lymphoid leukosis. Most people have birds that get sick, many die, the rest get better, and they never have them tested to find out what was going on, thus continuing the cycle of infection.
While this may not be what your birds are carrying, I hope that this information helps. Knowledge is power, and lymphoid leukosis is clearly here to stay in the poultry hobby. Here is a link to coccidia dosing: https://www.backyardchickens.com/t/818879/updated-corid-and-amprol-amprolium-dosing The moderate dose for 14 days straight is what my vet recommended and what has worked for me. Otherwise, a good poultry vitamin supplement a couple of times a week (Vi-tal is cheap, last a long time, and is readily available at most feed stores), quality feed, and a clean, healthy environment does wonders to promote a healthy, happy flock (so that lymphoid leukosis does not have to be a death sentence for all birds carrying it).
If you want to be sure of what your birds are carrying, have a necropsy done. They are typically around $50 or less for a gross necropsy, and $100 for more detailed results.
Take care, and I hope your birds get better soon.
