I will say this. Luffy's crew isn't like Kaido or Big Mom's. Zoro was always close in strength to Luffy. Now post-skip seems to have changed this with Luffy getting progressively stronger abilities and unlocking stronger haki, however I don't think Zoro could be that far behind. Enma seems to be a big boost and the nidai kitetsu could bring the difference between Zoro and Luffy to what it was pre-WCI, especially if the kitetsu is tied to Zoro's asura.
Luffy's crew seems to be similar to Shanks's. It was stated in vivre card(I believe) that beckman rivals Shanks in power and during shanks's bounty reveal, the strength of his crew was highlighted.
Hence, I think its possible that Sanji could fight King, with Zoro playing a main role in the defeat of Big Mom. After all, Zoro usually fights the second strongest, and the second strongest of the Wano arc isn't King, but rather big mom, who so happens to be a swordsman.
Basically, by the end of wano, the SHP would have 2 low top tiers(Luffy and Zoro), 1 YC1(Sanji), and 1 YC2(Jinbe). Luffy would be the weakest yonko(or maybe second weakest if Kid becomes an emperor), but would have the strongest crew.