Many years ago I had a problem like this with CRD (very similar to Coryza) in my Barnevelders and everybody I talked to had a constant problem with sick birds. I remember reading something, I think it was from Kenny Troiano in the Poultry Press, and reading that coupled with the same frustrations you're experiencing finally moved me to the point of culling every bird except a cockerel and 2 pullets that never showed any sign of sickness. Plus the disease was spreading to my other breeds including my Ameraucanas.
So, long story short, I stopped ALL medicating. No more Tylan, various "cyclins", Duragard, or anything else. If they got sick and got to a point where it looked like they weren't going to recover, they got tossed over the fence for the coyotes. The only thing I did was put bleach in their water to prevent it from spreading in their water. I have continued to do so but more to keep the algae/mold from forming in their waterers. Even that hasn't stopped the algae/mold entirely and they get fresh water everyday!
It was either in 2 or 3yrs that I stopped seeing any sick Barnies. In the last 5-6yrs I've not had a single Barnevelder get sick. Last year or the year before i had a couple of Ameraucanas develop bubbly eye and a bit of runny nose. I immediately put them down and fed 'em to the coyotes. I've seen nothing since.