Well I guess I've to wait the end of Wano for Zoro being stronger than Ashura.
One Chapter before Zoro's main fight, Ashura will be stronger than him.
After that Chapter Zoro will finally surpass him, going from being Top Vet level, to top Yonko Commander level.
With Enma giving him massive Boost in Stamina, endurance, speed etc..
@Rivaille Our boy will go from being weaker than Ashura to reach or even surpass prime Oden level
He will grow faster than Coby