I don't want to freak you out. But did your vet listen carefully to your dogs heart? Coughing can be a sign of heart disease. The coughing is what finally made my vet realize that my old dog, Tiki, was suffering from heart failure. I have no idea why he couldn't pick up on that earlier. Shortly before he finally discovered the real cause for the coughing, he thought the coughing came from damage to the throat caused by a collar, so basically his treatment was "buy a harness"... So yeah. I don't know if my vet was grossly incompetent or if heart failure really is that difficult to determine. The earlier it's detected however, the easier it is to treat.

Prior to that (some months before the coughing got bad) she was often restless but not knowing why, the vet and I just assumed it was arthritis making it difficult for her to settle down at night because of pain. Reading about heart failure later on, I realized that restlessness can be a symptom of that, since lying down and putting pressure on the chest causes the dog discomfort. Being out of breath and less energetic is also a sign of heart failure but with Tiki that didn't really show until at the very end, which is pretty unusual. But she was always a very tough and energetic dog, so I suppose she just didn't show signs of slowing down until the problem had progressed as much as it did.

Again, I don't want to paint the devil on the wall and scare you unnecessarily but I just wanted to mention the possibility of heart disease, so you can make absolutely sure that your vet checks for that.

*hugs to Zeke*