luffy had more in his arsenal but zoro himself said ashura is his strongest move
Just like Luffy tried everything against Katakuri & needed to remember Rayleigh's Teachings
We have seen Limit of Post-TS Zoro
Now it's time to see what Zoro learns next
Asura which is a move from +600 Chapters ago being Zoro's Greatest Technique clearly means that he haven't shown a Power Up yet.
Just like Luffy too not being aware of this CoC imbued Attacks but now he noticed it & will obviously learn it & use it
So same thing can be applied to Zoro who might learn to control his Haki better & make all his attacks stronger