Do you think ISDS is actually stronger than that attack which Zoro used to defeat Ryuma?
It should be; ISDS is an Ougi (secret/ultimate technique).
I think it's something like:
Santoryu Ougi > top Santoryu techniques > top Ittoryu techniques > other Santoryu techniques.
Enma probably greatly boosted Zoro's Hiryuu Kaen, but it should also provide a similar boost to his other attacks.
Zoro is a Santoryu Swordsman, I don't see his top Ittoryu techniques (a sword style he's stated he's weaker in) surpassing his top Santoryu techniques.
IMO ISDS > 1080 Pound Cannon/(Upgraded) Nigori-Zake > Hiryuu Kaen/Shi ShiShi Sonson.
This is assuming the same swords and level of haki are used on them.
Preskip Sanzen Sekai > 108 Pound Cannon > Hiryuu Kaen/ShiShi Sonson. I think it should be the same for the upgraded techniques. Or at least I see no reason to assume Hiryuu Kaen is suddenly much stronger than attacks it was weaker than preskip.
Current Zoro is just
that much stronger than his Dressrosa self.
@Haoshoku's take is on the mark:
It sounds a bit wild because there’s still also Ashura to consider.. but yeah he’s clearly taking that next step. I don’t think Oda would have him cut part of such a large object with an Ittoryu attack if he wasn’t trying to show that Zoro has reached new heights.
Multi mountain level for “Base Zoro” as
@Cinera says
Then possibly Small Island for Ashura Zoro.
Its clear that he uses his own haki but with other swords Zoros haki wouldnt accomplish the same effect as enma
I don't think current Zoro could have pulled off such a destructive Hiryuu Kaen with Wado Ichimonji, and Dressrosa Zoro could not pull it off with Shusui.