Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you today.
I am very sorry to hear about the severe outbreak you are facing. Especially since it does sound like you have been very thorough and have tried a wide range of treatments already without success. Therefore, I have to say the best next step here would be to pinpoint the cause of their signs as opposed to just treating blindly. Since we have multiple losses, this does suggest an infectious issue (bacterial, viral, parasitic, fungal) or a shared exposure (ie toxins, water contaminants, spoiled hay, etc). And just to note, while I am sure you have already, do make sure to have a good look at their feed, water, and anywhere they have access to remove those exposure concerns her.
Now to narrow our differentials, you could have your vet take airway samples from affected cows. This can be cultured to identify what is present and what drugs it is sensitive to. Otherwise (and preferable in this case), we'd want to consider having an autopsy. This can be carried out at most vet schools, vet labs and agricultural colleges. They can use a recently dead (needs to be very fresh) heifer but we may even want to consider sacrificing a sick heifer to give us the best chance here (even have it put down at the pathology lab). This would be the most straight forward means of determining what is causing death in the herd. And besides gross analysis, the pathologist can take tissue samples to check for viral inclusions or mineral imbalances. And with a fresh body, we can potentialy take samples for bacterial culture. So, while not nice to think about, this tends to be the best and most straight forward option to get an answer to which of the above issues is causing our losses and allow us to treat against effectively.
Please take care,