The sword is special and dangerous, more so than it's wielder here.
It's Oden's aura that Kaido is sensing here. Not Zoro's. Enma is specifically emitting a dead's man Haki, basically. It has a power of its own and in the image below you can see - again - that it produce its own aura, which is, as far as I can remember, the first and only time that it happened in the manga. It's also the only sword which somehow can drain the haki of its wielder to make stronger cuts, which is basically "a magic sword".
Regarding Zoro's link with Ryuma, it is obviously extremely relevant. I do not believe he is the heir of the last Daimyo of Ringo, but he's definitively a descendant of Ryuma and Oda pushed the thing far enough that the two are basically look-alike. And when one knows Japanese culture (or pop-culture at least) you knows that it means something, which is basically that Zoro's that strong because of who is ancestors are.
And said ancestor is obviously far stronger than Judge Vinsmoke ever was in that case. We aren't talking about an average strong dude, we're talking a man strong enough that he alone was enough to make the WG back off Wano, at least according to his legend. That's who Zoro is the second coming of. That's why he decapitated a western-style dragon like Ryuma did.
To get back to Sanji, his situation is, just like Zoro, far beyond his control. He didn't choose to be genetically modified before his birth. He didn't ask to be Judge's son, and even today his power-up, unlike Zoro's or Luffy's which are just power-scalling tools, make him question it, himself and what he's becoming, which is all extremely interesting.
Using the RS was bad, the "humor" he had since the TS is trash
, but Sanji's current power-up is the most interesting we have seen in the manga so far, because it makes the character who got it question himself, wonder if he'll turn into his worst nightmare. It opens up a trove of possibilities for future stories. What will Enma open as a story tool for Zoro, or Luffy's mastery of CoC ? Basically none or very little once this arc will be over.
For once, Oda took the time to build up this power-up and explain why Sanji isn't just going "Haha, I'm stronger now and I'll kick your ass!" And he even gave him an opponent who knows of Judge and his scientific experiments, while being himself a deadly scientist. He has tied Sanji deeply to MADS, and MADS has been tied to Vegapunk, the greatest genius ever in OP apparently. And MADS was comprised of a man able to create horrible pandemies as a hobby, one who loves to create weapons of mass destruction and one who wants to create the perfect human weapon/army.
Those are all extremely interesting elements which will be picked up in the future, which is great.
To sum up my position : Oda ruined Sanji more because of his pervert jokes than with his power-ups and actions in the post-TS era (save perhaps using the RS in the Flower Capital.