Oh you poor thing, so stressful. Your set up looks good, and if you are going to the vet - he must of questioned your feed? If he is happy with it, I would not worry about it.
Getting started can be a big learning curve, and while chickens can live a very long time, a lot of times they don't. A lot of times it is genetic and not much you can do about it.
I don't think you will want this advice - but I would consider culling your poorer doing birds. Poor doing birds get sick much easier, and then they can kind of harbor it in the flock, and any stressor can bring it back out into themselves and then the healthier birds.
Medicine is always an option, too. But culling is an old time animal husbandry technique that has long term benefits.
Mrs K