When people put aside their preconceived opinions on Zoro and bother to pay attention to his character, they tend to bring out Zoro's sense of sacrifice or his loyalty as his main character traits.
For my part, I think rather that it's actually his gratitude who served as a common thread to build him as a character :
Zoro & Kuina relationship
In his quest to become the WSS, Zoro has challenged Kuina to a duel countless times to the point where he nearly matched her level.
At Kuina's death, he recognised it was thanks to her that he managed to become stronger. All these fights they had wouldn't have been so intense if her too didn't have the dream to become the WSS. It's as if it was her will to make her dream true that made Zoro's one more possible now.
So, as a token of gratitude, he decided to take her sword and carry her dream as his own since fulfilling her dream first would then allow him to reach his.
So, besides showing gratitude to Kuina, Zoro also showed compassion towards her ... despite her implying at their last duel that it was only thanks to his physical advantage as a growing man that he became this strong.
It's like she was actually saying that all the efforts he did until now are utterly useless, making Zoro completely confused as he didn't know if he actually became stronger or not.
That's why he made the promise that one of them has to become the WSS : to raise Kuina's standards.
By raising Kuina's standards, his fights against her are more meaningful as she's following the same path of progression as him. So she can't use the sex inequality as an excuse to belittle Zoro's feats and hide the fact that he's becoming better than her.
Because in the end, that's what Kuina was all about : an haughty, selfish, insecure, coward and jealous girl who prefers to run from hardships rather than accepting it and moving foward.
In fact, she did commit suicide ("falling from stairs" is a japanese toned down expression of saying it) of despair to not become the strongest despite that she beat Zoro more than 2000 times !
But let us think a moment what Zoro felt and thought when he realized that she killed herself just after his promise with her : "if someone strong enough to beat me more than 2000 times had to get rid of her life because she lost hope to become the strongest, then ... what about me ? I shouldn't even have lived then, right ? Being dead is worth even more than what I am, is that it ? All the efforts I've done, all I've sacrificed to accomplished my dream ... are you telling me that I went through so much pain and hardships for nothing ? Shit !"
Zoro was betrayed by a girl, to whom he confessed his feelings, because she was insecure of her own strength as a woman.
He ended up being traumatized by it : that's why he was so suspicious of Nami, Vivi and especially Robin at some point and why he always spared Tashigi and Monet whenever he had the opportunity to kill them.
But despite all of that, he managed to overcome his despair (showing great willpower) and forgive Kuina for it since it doesn't change the fact that he did became stronger thanks to her and he still considers her as his friend (maybe he understood what she was going through and felt sad for her).
Zoro & Luffy relationship
There's a lot more going on with Zoro's sense of gratitude (with Luffy this time).
It was thanks to Luffy's dream that Zoro was able to find the path that led him to all these mighty opponents and that he had the opportunity to challenge the greatest swordsman in the world.
But Luffy also found for Zoro a stage where all the spotlight in the world will be on each of the feats he will accomplish to make Luffy, The Pirate King !
Zoro will derive greater fame from this journey with Luffy than if he had continued his adventure alone.
That's why he was ready to sacrifice his dream for Luffy to Thriller Bark since it was thanks to Luffy's dream that Zoro was able to pursue his.
In fact Luffy's dream ended up becoming Zoro's dream and suddenly he feels a deep gratitude for what Luffy has done for him. It's precisely this gratitude and recognition sense in Zoro that allows him to always become stronger !
Therefore, it is thanks to the links he has with other characters that he is growing stronger (although he is very arrogant at the same time 😅 but this is what makes the character more subtle than it seems).
Besides, I imagine that there will be something similar to Wano where his sense of gratitude will be further emphasized.
In short, Zoro is not an empty nor bland nor one dimensional character as the One Piece community loves to claim. He, as a character, touches on many thematics at once and while being introverted (unlike 95% of One Piece cast) he nevertheless conveys many complex feelings and emotions.
