Sort of an update, the Welsummer I mentioned has since stopped laying (last egg was probably beginning of July) and gone into molt. Assuming she makes it to next spring without issue, she likely won't start laying again until March or April. So it's a very abbreviated lay cycle for her at this point.Im convinced you hit the nail saying these two things.
1. Genetics (with genes that are not deformed for max egg laying or inbreeding for prize winning standard looks).
2. Chickens who have a short laying cycle are getting older (in general). Their bodies recover better from the egg laying season during the longer winter stop.
I also have a younger dud hen that either never laid or laid only a couple eggs (but most likely has never laid). I am curious to see what effect that might have on lifespan and overall health.