Welcome. As heart-breaking and in my opinion criminal as it is, you have the factory farm egg industry to blame for the issues you are facing with your girls. In the wild, hens only lay about 20-eggs per year. The factory farm industry over the past several decades has abused the hens to lay 200-300 per year.
Then, they get rid of the hens at 18-months, and not nicely.
Even though you are giving your girls a good life, they come from a line of production breeds that make them lay way more eggs than their poor little bodies can handle. The abusive increase of egg laying volume that the factory farms made has also resulted in many health problems. As one example, a hens liver restructures when she comes into laying. A high volume of eggs is the biggest factor leading to liver hemorrhagic disease which most of the time results in sudden death. I try to help their livers by adding milk thistle to their feed.
As far as ceasing laying early, this is also a result of being bred to lay too many eggs. Hens, like women, have a certain number of eggs.