Nothing suggests Akainu is way more powerful than the other admirals. Sure, he is stringer than then but by just a small margin: a tiny tiny bit stronger than Aokiji (beat him after a 10 days fight), most likely stronger than Kizaru by a small margin too, probably stronger than Fuji but again just by a bit.. and I hope Greenbull somewhat is clearly weaker than the others admirals otherwise one day we will have a massive meltdown somewhere, bugger than what it needs to be really. But everybody reaps what he sows.
Then about Akainu being an arc main villain at this point it is already too late. Kizaru, who basically is his equal, was a secondary antagonist in this arc compared to the 5 gorosei, even if it is true that he decided to stay down of his own volition at a certain point. But in the end Luffy was taking him and Saturn together when he was pissed so the admiral level is already behind Luffy. And it couldn't be otherwise since he dealt with Kaido the last arc.
I don't know about Sanji but Zoro, even if he actually struggled vs Lucci when Luffy didn't, is already ready to face an admiral at this point. Last arc he dealt with King and had his rooftop fight, that could already be compared to a fight with an admiral. This arc he fought Lucci (and didn't use any fancy move on screen) and then went on fighting while Lucci was spent. Anyway the next tier after the YC1 Zoro defeated in Wano is of course an admiral, or Oda could make him jump directly to yonko level like he jumped from veteran lvl Pica to YC1 King.. Admiral space is shrinking, they could keep up if they actually improved with time but that is something Oda gives only to the SHs (and BB). Some other guys like Lucci, Croco and other for sure get power uos with time but you can bet they will never again be comparable to Luffy since he already beat them (just see Luffy and Lucci).