Yea but zoro let him loose this time. no need for shushui to suck anything.
That's not how it works. Zoro letting lose with shusui would just result in him using more coa. Enma Sucks much more than what zoro can give at a time. So letting lose with enma means getting almost all your haki used.
Basically think of it like this. with shusui, zoro letting lose means he will use all his coa.
With enma, Enma will suck all his coa, and when its not getting coa anymore, it will tap into coc, which zoro has the potential for, and use it to coat the blade. Thus allowing zoro to use advcoc. This is just an example.