Luffy used to be "a kid that wants to climb to the top".
Little by little he became "the chosen one". Everyone around him is important to some extent, so he automatically becomes important too.
Grandfather? Hero of Navy.
Father? Most Wanted Man in the World.
Brother? Commander of the strongest man's crew.
The "D"? Destined to fulfill some ancient prophecy.
Laugh Tale? Actually became a place with a timed prophecy, instead of a really hard place to find with a poggers treasure.
Artificial ways to keep the character in the grand scheme of the world. I never really liked it too much because it felt "cheesy".
Mix all of this with 20+ years of the same character, with no signs of change, despite ALL that he endured (losing his crew at Sabaody, losing Ace, and so on), 0 maturing, many things that once were positives for the character, start to feel like negatives, simply because they shouldn't exist anymore.
About Zoro, I agree with your analysis, but I stray away after timeskip. Not after a single moment, but a piling of little narrative decisions that made me feel less invested on the character as a whole. Being a strong character in a adventure/battle shounen, to me, simply means this character can have more panel time to contribute to the story, wether he accomplishes it or not, in a satisfactory manner, is yet to be seen. In Zoro's case, his progression changed a little bit. He is not really "learning" anything new. Right now his progression is tied to forging permanent black blades. That's it. Enma was just an artificial "powerup" to replace Shuusui, that was ALREADY blackened. What I dont like about Enma, is a few things: 1- I hate Oden's character. Seriously. Nothing tied to this character seems good to me. It's like a contaminated apple that spreads its venom to the rest of the fruit basket. The scabbards feel like less sympathetic Shandians, Hyori and Momonosuke are retarded and annoying respectively, Oda decided to put SO MUCH importance in Wano in particular, that I simply can't bring myself to actually care about it. 2- Nidai Kitetsu was introduced at the beginning of the arc. Zoro has shown interest in it. Zoro was never "yo, I want the most broken ass swords in the world", he was always more like "yo, that's a fine sword, and I need a sword. So I'm gonna take it". It was like that for Sandai, Yubashiri and finally Shusuui. Him showing interest in Nidai, and saying "Ok, I will return Shusuui, but I want that Nidai Kitetsu that I saw earlier", he are stuck with the "ONLY ONE sword to have injured Kaido", despite Ame-no-Habakiri being in the picture, already wanking this piece of garbage blade.
With Enma out of the way, back to the character itself. Zoro came back from the timeskip extremely strong. Not a bad thing per se, but it starts to feel a little forced when you consider that other characters simply didn't have near the same growth, wether it be for the sake of gags, hyping other enemies, or simply keep doing the same powerup asspulling every arc. Zoro is THE ONLY SH, that felt like actually trained for 2 years, instead of 2 weeks. It gives the vibe of piling up feats for a single character in a similar way happens with Luffy. Besides, he is kinda back to simply wanting to be "the strongest there is". All the elements like Kuina's promise, willing to put his dream aside for making Luffy PK all feel non-existent. He is back to prior Thriller Bark, but now he is more overpower.
While I dislike Luffy's LACK of progression, with Zoro I feel there was actually a REGRESSION. That's why I can't bring myself to care about him much. Him killing 10 yonkos by himself is just...."ok, he is strong. What else is there?"

At least I still like his gags, because they contrast very well with his "serious and smug" attitude. And they are not some japanese pervert fetish.