Oh no! I am so sorry to hear this
May sound harsh but to live in my tank, the fishes have to be able to survive and fight off ich. Its just not avoidable in my tank and has been there going on three years now.
I have yet to loose a fish to ich in the display though, only in QT.
Right now I monitor it closely and as long as there is no scratching or signs of stress, I just feed well and keep the water clean. It seems to come and go. Before adding my Powder Brown, I think I was getting way ahead of the ich--he seems to be stressing the tank enough now to cause a new outbreak.
If a fish were to ever get bad enough to need QT, I would probably Qt them and sell. Replace the fish with another who can hopefully fight it off. Wouldn't be easy to do but like you said, if you QT them and put them back, they will most likely just get really bad again.
Its such a hard thing to deal with
Good luck!