https://pokemondb.net/pokebase/271667/whats-the-weakest-ember-that-cannot-ko-kartana?show=441292#a441292
Isn't Smeargle the weakest user of Ember? You could go the extra mile and make a Bonsly Mimic Ember (it has the lowest SpA in the game). I don't remember if Beast Boost can be Skill Swapped, but giving Kartana Thick Fat would be an option if it was. Also you could do Guard Split shenanigans to increase Kartana's SpD, but it does feel a bit non integral. I'm not sure whether the answer would change, but it is stuff to consider.