“Shallow”? Zoro literally trains, sleeps, drinks, is very loyal and has a stupid gag that should’ve been fixed with basic observation haki and the uncanny ability to be the only swordsman I know who can’t properly cut off a limb from an opponent
To be honest Zoros character right now seems to be that but if you holistically understand Zoros, he always has been the most fleshed out and engaging character.
Pre Time Skip each arc we got something different from Zoro where in his introduction arc he introduced Zoro as demonic villian from outside perspective but when we got to learn more about Zoro, we see that he is very kind-hearted and generous and turned himself in to save a little girl.
In that situation he showed situational awareness, he understood consequences despite being strong enough to kill Helmepoo and Morgans, he chose to keep his imagine clean and prevent any trouble from coming into village until he learned that Marines were always planning on double crossing.
Right there we got to learn trigger of his motivation to become WSS which isn't just one sided ambitious desire but rather a promise he has to keep to his dead child-hood friend. That right there makes Zoro more engaging than Luffy to me as we still don't know why exactly Luffy is always randomly screaming "I am gonna be a pirate king", he just thinks pirates are cool and there is that.
In Buggy Arc we got to see more of his tenacity and his dedication to his commitment where he saved Luffy despite open wound that could have killed him. He didn't have to go above and beyond, Luffy doesn't owe him much, he passed him swords and in return Luffy got a crew mate stronger than him.
In Sanjis own arc, we get some real life lessons of failures happen but in the midst of failures we shouldn't break down, instead keep going. Imagine Michael Jordan quitting after losing one HS Basket Ball game. I can easily resonate with that as journey to getting brown belt in Jiu-Jutsu wasn't easy, being as good at tennis as I am rn wasn't easy, getting my first job wasn't easy, there were failures and rejections along the way. Zoro as a whole has always been written in engaging manner, something Oda struggles with Sanji and Luffy where their dialogues always come off as surface level and basic. Zoros scars on the back dialogue, him talking about if he gives up there all his promises, his dedication resolve all in vein. We never get that kinda engagement from Sanji or Luffy, just random cool one liners here and there like "Mr Prince"
In Alabasta his character was fleshed further where Oda even gave nuances to his fighting style with him evolving not just physically (learning Shi Shi Son Son) but also mentally and changing his out-look on battles where he initially only cut to kill, he now cuts to protect who are dear to him. And just the way Oda took us on deep journey to get to that point with Breath of All things, his teachings, him questioning his moral compass is once again the kind of engagement we never get from any other Straw Hat. Maybe Nico Robin or Nami at times but not quite like Zoro.
In Skypea he was explored further in many ways with his out-look on men attacking women, his take on Gods, but generally Skypea was more of a combat arc for Zoro, there were still many engaging dialogues between him and Enel, him and Nami Robin Chopper etc
Water 7 we got to see Zoro as a voice of reason and level headed when entire crew lost their shit at the idea of Nico Robin leaving, he had Luffy and Nami sit down at the roof top and make them properly asses the situation, thinking of possibilities and it is all something a normal human can resonate with. Almost any tricky unexpected situation may triggered the kinda reaction which will make us do irrational things but voice of reason is always needed.
There is a lot more that can be said about Zoro but Sanji to me always came off as a shallow character, and obviously with goal that I can't really get into. I mean I do love to cook but Oda hasn't even stressed enough on Sanjis journey to find all blue, and then Oda basically made Sanjis character worse and worse each arc peaking during his training where he literally changed his gender only to come back to die from Mermaid titties.
It is a very easy and logical thing to be a Zoro fan cuz he's written that way.
Obviously everyone sucks Post TS and that applies to Zoro as well. WCI is the only arc where Sanji felt like a real character to me..but once again he was humiliated badly in the process.
Luffy is literally not even a character..just a parrot doing same thing and repeating same dialogue "I wanna be a pirate king"