But like you said, Kaku could more than a 6-sword and an 8-sword fighters couldn't, so the number of blades that dictates the outcome, but who wields them. Shiryu doesn't need a sword with eight heads; he needs better swordsmanship.
There are too many Ittoryu style users in Zoro's path, they have to be different in some way.
Mihawk, Shiryu, Venus, Fujitora/Kizaru, all with the same combat style but narrative requires different challenges.
Surely, if we take Fujitora/Kizaru/Venus, the difference comes from Devil Fruits augmenting their swordsmanship allowing for additional abilities such as Gravity, Light and physical buffs among others.
We know Mihawk is the ultimate Hakiman so the final challenge is reserved for a test of haki but what about penultimate test with Shiryu?
He should represent the ultimate DF merchant in Zoro's path and that's why the focus of Blackbeard Pirates are Devil Fruits and sharing a theme with Blackbeard of multiple DFs, Shiryu's sword being fed a DF is logical, because no other has such characteristics.