Funny how people here kick the context to kingdom come here.... zoro performs Onigiri and says that he has to unleash more of enma and not that he has to exert more generally... and we can see how he learns from this when he first cuts kaido and even kaido notes to enma.
zoro imbues his enma with even more haki aka unleashing more of enma... which he didn't when he was executing onigiri on kaido.. which logically he has to exert more to control haki in enma coz it rapidly absorbs it.
if every other sword was imbued with the same amount of haki.. Kaido wouldn't just point Enma out having that Strange haki
Again just like shusui its illogical to say that he has mastered enma just days into its use.. with shusui there was an unbalanced of weight included in his 3 sword style which in this case is Haki instead of weight..
again there is no proof entailing that enma has been mastered but there still is evidence that he hasn't mastered the blade.
He said it himself that he'd be stronger if he did master the blade and the only thing he is doing right now is use enma primarily to hurt kaido by using its quality of absorbing more haki, him mastering the sword would logically mean that he would control the haki flow in enma instinctively like other blades and not using it for unnecessarily large haki slashes which leave him wheezing. bruh