First off, I'd like to suggest you check out Farmerboy16's thread,
https://www.backyardchickens.com/t/587948/rebuilding-my-farm , He dealt with CRD, found a way to start fresh and continue. It's a longish thread, but worth the read with the info he's provided and the experiment he tried and had some success with.
I am sorry for what you are going through. New chicken owners are more likely to not realise the importance of quarantine. Never bring in new birds adn add them to your flock until after a quarantine period has established that they are not ill. Stress will bring out illness fast, especially if the bird is a carrier. Always take keep new birds AWAY from the established flock, by keeping them in a garage, clean-dry basement cage... spare bathroom... Take care of that bird after you've already taken care of the established flock, this way you don't bring anything from the new bird to the flock.
Treating of the birds doesn't have to be a trial, and keeping a closed flock is always a good option, not allowing other chicken owners to come in and see your birds either. A lot of problems are actually airborn, or carried in by wild birds. The only way to avoid that is yes, a bubble, never letting the birds out of the coop in the sun or on the grass.
One of the fastest treatments for bird showing signs of CRD is getting Tylan 50. That is available at feed store, it's in the cattle section of meds. You can either give it by injection or in the mouth. .5 cc/ml for large/standard size birds. Once daily for 5-7 days. .25 cc/mil for bantams smaller birds. Same amount of time. If you plan to inject, get the finest needle you can find. insulin needles work. In the breast, avoid the crop, alternate sides daily.
LS50 powder, in the water, 5-7 day, some say as long as 10 days.
Tylan powder also, in the water, same amount of time, just be sure to make fresh doses daily and it's the only water to be available.
Oxine is another option. You not only mist in the birds faces, but you can clean/disinfect the whole coop to get the disease out.
If you decide to cull, but want chicks from the flock you have, please read Farmerboys thread, it tells you how to do it. Then get the vaccine and have it ready to vaccinate as soon as your chicks hatch. If you can not do this, pay the hatchery or find a breeder who will vaccinate all chicks you purchase. You may want to vaccinate for mareks as well, as that is even more devastating than CRD, since you can treat for CRD, and work at keeping your birds healthy even if they are carriers.
As for me, I do quarantine, and people should be prepared for problems when bringing in new birds. Sometimes a bird can be a carrier of something and the owner never know it. Some birds are better at building immunity, getting over it quick wehre you don't have to provide any antibiotics at all. When you go to swaps you have to expect that it can happen. It's a hard lesson, and one I learned the hard way when my neighbors brought home birds and two days later nearly all of those birds exhibited signs of corzya. They didn't quarantine.
Keep your head up. Don't give up if you really enjoy birds.