I apologize for my delay due to the holiday, but I'm getting back to you, no need for spam lol hm are you sure about Greninja? Unless I'm misremembering, my cousin just traded me his as he wanted to restart his game over. But no worries, I'm not sure we were going to be able to agree anyway. Thanks for the offer anyway!
Stealth Rocks is naturally learned by Lycanroc (and Rockruff) at level 29. Alolan Sandslash can learn Icicle Crash via the move tutor (and I believe on evolution). Both of these are in the Field egg group, as is Swinub.
Given it's in the field egg group you can technically access any of a Field Pokemon's egg moves. Because so is Smeargle. Provided you have a Pokemon which can naturally learn one of Smeargle's egg moves, you can just use said move against a wild Smeargle with a low level Pokemon (they can learn any move in their level up pool through relearner now) provided it outspeeds, and then catch the Smeargle after it Sketches it. Voila, you have your egg move.