Sorry to burst into this Welshmen. Perris gave excellent answers so far. ButIt helps a lot!
Last question: are you familiar with any treatments to give sick chickens or something to boost their immune systems in general? Medicines, foods, supplements, herbs, etc?
maybe I can add a few ideas and a response to your question about gheir immune system.
Healthy supplements for chickens I give my chickens now and then: garlic, apple cider vinegar, oregano. But there are more. There is an article on herbs in the learning center. https://www.backyardchickens.com/articles/top-50-herbs-for-your-chickens.77443/
Probably the number of eggs is fine for the number of chickens (more heritage breed less production hens) you have. But the following crossed my mind while reading about your ‘problem’ and I wrote them down just to be sure.
Did you check on mite and lice? Or worms? Especially red mites and a severe worm infestation can cause trouble in paradise. In some states the northern fowl mites can cause problems too.
And I wonder how much space (run space) the chickens have. Because if chickens who have little space they can get stressed and lay less because of that.
A good run for 6 normal sized chickens would be 24m3 (about 12 * 20 ft) .