Hi

I'm going to be Debbie Downer. Sorry...ish.
I sympathize for you, I really do. But do understand that standard protocol for infectious respiratory diseases is to cull the whole flock and, depending on the pathogen, to register the outbreak with the USDA. If you go to vet and they identify a pathogen that is required to report it then the vet will contact the state veterinarian who will then report it to the USDA and whatever protocol is in place to deal with that will be utilized.
Very rarely does it come down to this, but it is a risk you'll have to be willing to take if you get testing done.
Second, IF it were a bacteria and the antibiotics you were using were effective then it could interfere with a positive diagnosis. I doubt this will happen, since the treated birds didn't recover and the signs all point to a virus making its way through each bird.
If you wish to seek them out, there are some antiviral medications available for poultry. But I don't think you should treat for what you don't know you have.
Get the diagnosis then treat appropriately.