I am really sorry for your loss.
Your coop and run are WAY overcrowded! You are close to industrial standards.
And at half the amount of space that is generally recommended for home keepers. Of course there are no magic numbers and they are just guidelines.
Where did you get the 5 birds that you added? Maybe you brought something in with them.
If your dead bird is refrigerated you might send it for necropsy. It is usually low or no cost with many state ag departments and universities. It should give you a definitive answer as to the reason for death. Here is a link that might be helpful in that regard...
https://www.backyardchickens.com/t/799747/how-to-send-a-bird-for-a-necropsy-pictures
IMO.... you will not be getting better until you improve the housing and run situation. Aside from them being crowded, it's also harder for you to keep clean, I imagine. And I am not saying anything about you... I just know that parasites and disease thrive in over crowded conditions. And though they may always be present they only become an issue when there is an over growth of them.
And you might be thinking you are doing what's working for you.... but it isn't working anymore. Yes we all have to do what is right for us. And you may be able to keep up better than me. And birds can be happy in minimal space. But you have lost the balance that you had and need to get it back. How much roost space? Enough for everybody?
My real concern is those new birds you brought in.... This is your biggest culprit for real.
It may not help, but I would give them some electrolytes and probiotics as a little boost to help the rest fight off whatever it is. Simple homemade recipe is...
2 cups warm water, 2 tablespoons brown sugar, 1/2 teaspoon each salt and baking soda. (white sugar OK) Serve full strength. Do not provide supplements for more than 10 days. And I usually add a couple drops of Poly Vi Sol with NO iron, baby vitamin drops. It must have the B's, not all baby vitamins do.
That is just electrolytes. For probiotics some people use raw, unfiltered, apple cider vinegar WITH "mother".
I think like 2 tablespoons per gallon. I don't know if the two are effective when mixed. They also sell some stuff at the feed store called sav-a-chick that has electrolytes and probiotics in it.
I can't remember if any of your chickens were roosters?
For me the flock is an ever changing dynamic and I must adjust to meet their needs. So even what is working for me now might not work next season. Point is... I'm not trying to harsh you about space or practice. Just trying to help you resolve your issue.
Come on flock, pull through!