I'm so sorry for your losses. I lost several of my birds when they were between eight months to a year old. Unfortunately, it seems like that is a time when a lot of health issues really start to show up.
When trying to solve a medical mystery like this, the first thing I would do (which you might have done already, and if so, you can skip this step) is to pick up each bird and give them a thorough health check. I would check their body condition. How do they feel when you pick them up? Lighter than usual, heavier, the same? How does their keel bone feel? Body condition can provide a clue as to the bird's health. I would check eyes, nose, inside the mouth if you can manage although that one can be tricky even with multiple people. How does their comb and face look? Paler than usual? Tinged blue or purple at all? I would feel their crop to check for anything there that could indicate a blockage or sour crop or anything similar. I would also check to make sure that their crops are filling and then emptying properly overnight. I feel their entire body, checking to make sure that there are no unusual lumps, bumps, swellings, anything like that. I check the feet for anything unusual.
I also check to make sure that their abdomen is not larger and more swollen than it should be, which can be an indicator of salpingitis or water belly. I look for any signs of mites or lice, both on the bird (under the wings and around the vent especially) and in the coop/run. How is their egg laying? Are they laying or have they stopped? Have they had any issues with soft eggs or lash eggs? Egg binding? Are there any signs of prolapse? Have they had any other health issues in the past? (This does not always mean there will be an issue, but can be an indicator - one of my hens had a prolapse several months prior, another struggled with laying and had also prolapsed, and the third struggled a lot in the heat.)
The next thing I would do would be to inspect the environment. Could they have access to anything that would make them sick? It could be feed that spilled and then started to go bad because of dampness, or anything else like that. Are there any signs of parasites in the coop?
Best of luck. I hope you are able to figure out what is going on.