Before I even answer this question I need to explain what ryu is. Some people don't understand. Ryu is Wano citizens term for haki. The only difference is most of them are swordsmen or weapon users for them to be able to properly use ryu or haki with their weapon or fighting style they have to learn flow/barrier haki so they can add it to their weapon. Flow/barrier haki is the ability to flow haki from the body.
Zoro first used ryu against Mr. 1. Yes, he has had that power (ryu) since pre time skip.
First, Zoro having enma using doesn't automatically mean he can Kaido. When Zoro first attacked Kaido he didn't cut him. That alone proves the ability to cut Kaido comes from the users (Zoro), not the sword (enma). I almost believe Oda did that on purpose to squash the belief enma is the reason he cut Kaido.
When Zoro used Dragon twister he cut Kaido with all three swords. That is a 3 sword attack. If he wouldn't have cut Kaido with all 3 swords then he would have commented on it. The manga even hints at Zoro ability to cut Kaido way back when he cut the dragon during punk hazard. That's why that feat is compared to Ryuma's feat in the official one piece magazine and even ask if Zoro will repeat Ryuma's legendary dragon cutting feat in Wano.
Enma doesn't increase the strength of Zoro's haki, it just forces him to release larger amounts of haki. Zoro's haki strength would be the same with or without enma. Enma does not retain its previous users haki or store haki.
Yes, Kaido and Big Mom both comment on the sword because it is a unique sword. I have already explained that.
When Zoro cut Kaido with his dragon twister Big Mom stated Zoro cut Kaido. While Kaido stated he felt an unfamiliar haki. It wasn't Oden's haki since Kaido is familiar with Oden's haki. That means the haki he felt was Zoro's haki.
Zoro would be stronger right now with shisui because he fully mastered that sword.
His battle against Pica showed us a small percent of his true power.
We didn't see the true strength of that attack because he didn't need to use billion fold world to cut the golem. The second attack Zoro used, which was a none name attack, cut a large section of Pica than his first attack.
I have already mentioned he cut and killed a dragon with shisui.
Swordsman grow stronger with new swords but that is after they master the sword. That comes from properly controlling the sword. If the user cuts too little or too much with the new sword they haven't mastered. Zoro hasn't fully mastered enma. He is stronger with enma then he was when he first obtained it but still has ways to go. Remembering, what true makes swordsman strong is their ability to cut what they want to or not cut.
Zoro without enma but with shisui would perform similar.
PS: The reason some believe Zoro can't do much to Kaido without enma is because they fail to understand ryu/haki is the key to cutting/hurting Kaido, not enma.