I agree stress is the biggest problem in this hobby and being most of us can't read the expression on a fishes face, it's hard to tell if it is stressed. But I'm telling you that almost all fish in captivity are stressed, and it is our job to keep the tank as stress-free as we can.
I think eliminating stress would do much more for the health of the tank than trying to kill all the pathogens.
This seems very simple, common sense to me, and I think all of the very old, non-quarantined yet disease-free tanks all have reduced stress in common.
Quarantine, unless it is done in a tank as large as your DT and set up as well, is extremely stressful, which is why so many posts are about sick fish in quarantine. Just look on the disease forum. It is the act of quarantining that is actually causing these diseases to attack our fish, and if medication is used, that compounds the problems because we can't know how the fish feels after being in a bath of medication.
I know how I feel on medication, and I never drank a medication if I had a skin problem, but most disease threads are about parasites or some sort of skin problem. When we try to kill or cure those "skin" conditions, we are forcing the fish to drink all those poisons, and we are hoping that it will kill the parasites. We can't rub copper on a fish to try to kill a parasite.
If we get a rash, we can put some calamine lotion on it, but try drinking it.
Copper is a toxic poison to us and fish. Yes it will kill parasites, but apparently less stress also kills them and their DNA.
The tank transfer method also has to be extremely stressful because fish need to feel secure, and keeping them in a small tank with PVC elbows and transferring them constantly must be terrifying for them unless they work in Home Depot.