I honestly thought Zoro would be called the Monstrous Samurai considering his whole character embodies the bushido code. Instead Luffy gets that comparison despite his character being the complete opposite of a samurai. Personally, I am still confused by that comparison. I really thought I knew how to read.
Pretty sure Kaido mentioned the "monstrous samurai" being capable of reopening his scar when the Scabbards failed to do so then Zoro created a new scar. I'd say Zoro was the monstrous samurai, Oda just didn't feel the need to repeat it since he mentioned that the previously mentioned accomplishment is what such a person would be able to do at bare minimum.
In my opinion, Luffy wasn't the monstrous samurai, he became a legend of his own of similar renown as Ryuuma. His fame and heroism is now comparable to Ryuuma but the only time I recall a monster samurai or anything like that being mentioned was when the Scabbards failed to reopen Oden's scar which Zoro went beyond by creating a completely new one and being acknowledged by Kaido.
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I'll be honest, a lot of stuff was underwhelming but for the most part, Zoro was treated quite well. Zoro vs Killer, Zoro cutting down Apoo, Zoro holding back hakai momentarily, Zoro defeating King, pretty nice list.
I'd say there's room for Oda to revisit some of these plotlines like the Ryuuma visit if he incorporates it into an interesting scene with a flashback but ultimately Oda shouldn't rely on that too often. Other stuff I feel is nit-picking like the rounding up the samurai thing, it wasn't that big a deal. Other stuff is indeed underwhelming but characters are allowed to disappoint, Zoro wanted to cut down Kaido but stuff doesn't always go the way a specific character wants and he'll just have to channel his frustrations into improving.