I feel really bad that you're going through this, when you're trying so hard to take such good care of them.
It does sound like there's something wrong in your flock, but I just don't know what it could be. Since they're different breeds, it doesn't sound like a genetic issue would be likely. I'd consider having a necropsy done, to try to find out what's going on with your flock.
I don't know what types of diseases could contribute to an early death, without showing much sign. Organ failure could be a possibility, but I'm not sure what could have contributed to that. Different toxins in the environment or a contaminated feed could do it. That's all a long shot, though. It would be something that wasn't normally a problem.
Is there any way they've gotten too many electrolytes? I know a couple of times, when people accidentally mixed their electrolytes too strong, their chicks died from it. The instructions were written more for a large poultry operation, where you make a stock solution with water and then added a certain amount of that stock solution to more water after that. If you gave extra electrolytes, even at the correct dosage, for a long time, I wonder if that could have been a problem for them? If they already had organ weakness from something else?
I really hope someone comes along that has some additional ideas for you. Hang in there. At least they are enjoying life while they're with you. Commercial chickens have even shorter lives and they aren't having the great life your chickens are. You've saving some other chicken from that life, even if not all your chickens have lived to be ancient little hens.