I think the key to successful atty maintenance is preventive.
All of mine get thoroughly cleaned before they've vaporized less than 1 ml. of
e-liquid. In most cases, it's
much less than 1 ml. Occasionally I'll see a slight discoloration before I clean them, but most of the time they look and test much like a new atty before cleaning. After cleaning, of course, they're squeaky clean. If you let the gradoo build up, then the cleaning task is much more difficult and fraught with failure, in my experience. If the glop is on so thick that you need a jackhammer to dislodge it, and you need the lips of a trumpeter to take a drag, then the atty is beyond salvation.
I've pretty much switched to cartos now and usually only use a cart/atty combo to evaluate new e-liquids. But before I switched to cartos for daily
vaping, I passed 50 ml. or more through many of my attys. A couple of them had to deal exclusively with fairly goopy VG "tobacco" flavors. I have yet to lose an atty -- they all still function perfectly.