They are not machines and egg laying is just a bodily function, and it shifts and fluctuates like any bodily function. It can be slightly influenced by the diet, but not as heavily as some posters indicate. It is rather like drinking a soda once a week is not going to give osteoporosis.
You do get thin eggs, and shell-less eggs. You can treat it or not treat it and in a while it will go away.
You are not going to keep these birds alive forever. They do tend to live about 2 and 1/2 to 3 years +/-. Do the best you can by them, and don't worry about it. As my granddaughter say, 'well that is tough, but now we can get chicks!'
I think your feed is fine. I think this year they will slow down during the dark days of winter, but may still give a couple of eggs per week, several will probably live through the winter, but some might not. A lot of people seem to like real old chickens, but they suffer quite a bit too.
Give em a good home, enjoy them, but realize they are not real long lived birds.
Mrs K