I am sorry for your loss. Sudden death without obvious explanation is something any chicken keeper has or will experience sooner or later. Commercial operations where the birds are fed entirely on commercial feed expect it: this from a breeders' guide
"Flock management
Dead birds can be a source of bacteria multiplication, so they have to be removed from the house every day and be disposed of in a hygienic way."
It sounds like your birds are happy and healthy so I don't know why you're worrying about fat. Fat birds don't fly and run.
If you are interested in my feeding regime, I wrote an article about it recently, and the article was well received by the judges - indeed, I see it's item 1 on the front page of the featured content this morning!
https://www.backyardchickens.com/ar...eat-tears-a-calculator-or-deep-pockets.78655/