I like you have a very large collection of breeder rats and have been doing so for just as long.

Here is what I do - usually only 1-2 per winter will get sick.

My breeders who get sick - are treated and given "time off" till they are healthy again and back to breeding weight (assuming they lose some while recooping)

Feeder Size that get sick (I will make a call based on what the sickness appears to be) are feed off without meds.

I have 5 "pets" that are no longer breeders but any other retirees are euthanized.