I used to own a Richard #7 Kamado, and a friend owns a #5. I helped with stuck dampers on both cookers. If POSK is too blue for you, try BTK ("Boat Thief Kamado").
Everyone here was nice to me, and I ended up buying a 23" Ultimate nine years ago. Like moving from a rusted out crop duster to a Lear jet. I had to convince my wife to spend the $700 on the Richard BTK. She convinced me to buy the 23" Ultimate from Dennis.
Yes, penetrating oil and a homemade strap wrench. Somehow you have to apply sufficient rotational force without hurting yourself.
Once it's apart, find yourself a stainless steel bolt and nut of identical gauge. Then, completely rehabilitate both the existing nut and bolt, by repeatedly working the new, clean hardware up and down the old hardware. Use appropriate solvents to suit the problem. In particular, it is possible to work a nut further along the existing bolt than is possible with the damper itself attached. Cleaning further gives you a margin, so completely closing the top damper never reaches the remaining corrosion.
With this hardware, you can clean annually, and avoid future problems.
If anything goes wrong, you can make repairs yourself. Your cooker will of course lose any value as an authentic antique, because no one can find the same crappy materials that Richard used at the factory.