I do care about the long term health of my flock and their happiness! Even though I will be raising meaties (I am also appalled at the chicken industry as a whole), I still treat them well. Some are pets. And I will be breeding my own. So the parents must be healthy if I wish to maintain any sort of self sufficiency. And when we do eat some, I will know they weren't raised in terrible conditions after being debeaked as you say at the hands of abusers.
I would say "all flock" with oyster shell available on the side at all times. I agree that layer feed basically adds calcium which your hens may not need while they are not laying, say during the winter. And I think winter lighting to increase egg production would be one of the worst offenders to decrease life span (IMHO).
You might research "fermented feed". Many on here swear by the benefits proven by their own experience. I must think their info makes sense because I started doing it a while back. My birds are all young right now so I don't have long term results, but others do. They are also thriving. It's a little more work than dry feed but I don't mind. It's supposed to increase vitamins and absorption of nutrients. Since I started, the poo has been the most noted difference. It's only runny every 10th or so, but otherwise nicely formed!
You could also check into apple cider vinegar with "mother" for their water. There are a lot of brands that will work, so you don't have to buy the expensive 1 if you do go that route. A lot of people on here also swear buy it. Search those 2 things to help improve your flocks longevity, and see if it makes sense for you. What works for some may not work for others.
It's awesome that you are looking for ways to provide a healthy lifestyle for your flock. Hope this info helps point you in the right direction.