Using the dragon as a last resort means we actually have to go on a adventure with this guy.
Which is a terrible idea. He's not going to lose to anything else at this point. Killing the other kingdoms would be nice. But not as nice as getting away from this jerk.
Send him against the dragon first. It is the only chance we have.
Our biggest problem is that we currently do not have a way to separate ourselves from him- while dumb, he's not quite
that dumb, and he's resisted all our "you go that way we go this way" plans. And while powerful, I don't know as he's a juggernaut or anything- we'll figure out a way to kill him sooner or later. And even if he
is an unstoppable killing machine with no equal on the planet, then we pretty much have to have him under our control so he isn't under the control of our enemies!
Or to put it another way, the plan to have him go kill the dragon has a few different outcomes- either he dies (win for us,) kills the dragon (wash, since we're neither better nor worse off than we are now,) or gets the dragon pissed enough to come kill us (big loss for us.) Operation Crown of Bullshit has the potential outcomes of him dying (win for us,) him destabilizing one or more of our neighbors before dying (big win for us,) or him beating all our neighbors and surviving, in which case we're in the same situation we're in now, but less several enemies. With any luck, we'll have built up enough "good guy cred" with our "cult destroying adventure" that he'll believe whatever story we tell him to make him go fight the dragon without us.