Just coming back to close out this thread. I euthanized the fish on 12/23 after a long slow decline. It progressed to the point that the fish was laying flat on it's side and was getting covered in rot and mold, so I put it out of its misery. None of my treatments made any impact. I do believe this was bacterial - just not something that the meds I had on hand were effective against. Maybe there are other antibiotics out there that could have worked better, but who knows.
It's frustrating - this fish was one of the biggest and healthiest of the bunch. There are others that I've had issues with, including a runt, that I would have expected to be the first to fall if there was something wrong in the tank, but they seem fine. I just don't get it. It's really unnerving to see a perfectly healthy fish go south all at once like that.