Encore is a good option also that prevents things from setting up on you. Taunt is kinda lame because faster shit can still get Dragon Dance, CM, etc.
Encore is a great move for smeargle. With encore and U-turn smeargle has a plan for fast sash leads, and for bulky leads. As bulky leads like swampert tend to set up stealth rock first turn, and encore can cripple them. Why not use spore? because you only get 1 spore and it's nice to save it for after they switch there lead out. Encore also means that smeargle is not set up bait once he has used his spore.
I don't think stealth rock and spikes should be together. Smeargle is not going to them all up together and it would be much more effective to have a separate SR user, that works well with smeargle.
Lastly is the max speed. I don't see the point in it TBH, I think it should be lowered to around 245, and put a bit more in bulk, giving smeargle some more usage if he has to switch back in SR and break his sash as he could potentially live a hit from a non offensive pokemon.
Also for a lead, dragonite is the next slowest pokemon, and lead dragonite has 200 speed. So from a pure lead perspective you are wasting at lot of ev's in speed, that could be used to bulk him up a bit more.
Taunt is not great on smeargle, he doesn't have time to waste turns with taunt and doesn't gain anything from it. Encore however can bide him some more time, so I would suggest encore personally.