OK, I think you have done everything you could. If the fish was mine, I would put her into the DT now. You have her long enough in the QT and went through copper.
I recently quarantined a fish showing the exact symptom (breathing hard) after being fine for a few weeks. I transferred her to another QT tank with freshly mixed salt water (100% water change
). She recovered on the same day.
There must be something in the water that we couldn't test that affect the fish.
Good luck!