If the vaccination or medication isn't working, why bother to use it? But I am a fan of non-vaccination, and natural medication, so I would say that.
But it's possible you're treating with the wrong thing, if you don't know exactly what they have. Do you breed them or bring in new lots, all-in all-out style? I'd expect some immunity to build up if you kept breeding the survivors. I can't offer any help regarding man made medications of the pharmaceutical sort because I don't use them, but I'm pretty sure you need to find out what you're treating to make the treatment successful if you are using them.
'Bronchitis' technically means any inflammation of the bronchi. A LOT of things can cause that. Since this is a severe case your birds keep showing, some in depth investigation is required. And more raw fresh garlic. Another natural treatment for inflamed airways, and all other mucosal membranes, is slippery elm bark powder; this is expensive but pretty efficient and can be a lifesaver, as like garlic it contains natural antibiotics, which come in dynamic form so the viruses, malicious bacteria etc can't become immune to them like they can to an unchanging single-ingredient man-made antibiotic.
Have you limed all the ground? If the soil's carrying some pathogen or its host, liming can wipe out a lot of things. I use the Agricultural lime they feed to animals for this. Sometimes the disease organism is a simple one to overcome if they're not overwhelmed environmentally by it, and liming can often help with this. More than just the coop needs doing if they free range. Mine do, and I've limed well over an acre around the pen, literally everywhere they go. Not just once, either. To begin with it was a few times a year for a few years straight before the soil started to recover its health. It wasn't a great place to begin with, now it's as fertile as anything.
I never have chick mortality with chickens to diseases, but I did have some chick mortality with turkeys to start with; I had some losses to Tuberculosis, (which I fixed), then Blackhead (which I've also stopped from happening now). With chicken babies, I expect 100% survival into adulthood. Sometimes injuries happen though. I don't operate a closed flock, don't vaccinate, don't use pharmaceutical medications, feed as naturally as possible, etc, and bring in sick poultry from other places to treat through diet alone and don't expect to lose mine to their diseases (and I don't) so it can't all be a fluke. I give raw garlic daily, most days of the week, and give them kelp as a multivitamin and mineral mix. I think most diseases can be overcome through diet. Our bodies are made to fight and cope and adapt, same with animals, and we need to be living in a somewhat natural state (mainly regarding diet) in order to give ourselves a fighting chance. Anyway, all the best, I hope you don't lose more. You shouldn't feel stupid, we're all learners, even those wearing the expert badges who've been keeping poultry for half a century.
