Welcome To BYC
I'm sorry to hear that you brought in disease to your flock.
Unfortunately, this seems all too common when you bring in birds from outside sources.
Culling is a viable option depending on what disease you are dealing with and what your keeping goals are.
Chicken diseases make birds carriers for life, affect production and egg quality and may affect the reproductive system in their later years. Depending on which disease you are dealing with, it may also be passed along to the embryo of hatching eggs.
Keeping a closed flock is the ethical thing to do if you have respiratory disease in your flock. No selling/giving away hatching eggs, chicks or chickens and not participating in poultry shows can help reduce the spread to others.
Also being practical and not wearing your chicken shoes/clothes to feed stores is a good idea as well.
Birds can become asymptomatic and thrive. Stress and other conditions like parasites, molt, etc. can cause relapses, so monitor during those times and staying top of parasites is good practice.
Remedies can be helpful in treating symptoms, so I'm glad you were able to give your birds some relief while they recovered.
I've never found a radio or noise to be a deterrent for possums and racoons. Short term possibly, but they become acclimated to the noise. Secured coops/runs/housing, fencing/electric fencing are all helpful ways to prevent losses, but even with measures taken, predators can still find a way in or if you free roam your flock, they will watch patterns and learn to take when the time is right. It's a constant battle. Trapping and dispatching predators is also another option if you find you have one that is repeatedly visiting and trying to find a way in.
Here's some information about common chicken diseases. Unfortunately, chickens get respiratory diseases, not colds.
https://edis.ifas.ufl.edu/publication/ps044