Wow, your "before" pictures shows the cleanest smoker I've ever seen. Perhaps you are keeping it
too clean? All commercial smokers have layers of black stuff that have built up. This is also true of commercial pizza ovens. I think this "patina" is part of the secret of producing great-tasting product. Even after only a dozen smokes, my smoker's walls were almost black. Here's video of what they looked like back five months ago, when I'd had it for three months:
Smoker Interior
If I took the same video today, all you would see would be pure black.
That said, I too have definitely had the unpleasant experience of having the smell from a previous salmon smoke "infect" my next smoke. My solution is what has already been suggested: crank up the smoker to max temp and let her cook empty for 3-4 hours.
Like you, I did initially obsess about keeping the walls clean, but I slowly lost that war. Short of using oven cleaner, I have no idea how I could get things clean down to the metal now.
However, I want the thing to function, so in addition to wiping it down after each smoke to remove loose debris, and cleaning the drip pans (easy since I foil both the drip and water pans), I do clean the racks, the rack supports, and the vent, the latter to keep it functional. My latest discovery is that my ultrasonic cleaner makes these chores trivially easy. See this post:
Ultrasonic Cleaning