After watching JoJo's Bizarre Adventure, specifically part 3/Stardust Crusaders, the reason Zoro is so boring became even more clear. He's the stoic badass, but with little to no traits post-skip to actually supplement this to make him stand out from any other stoic badasses. Characters who have the stoic badass characterization work best either when they're an obstacle to the main characters, or have underlying traits to redeem this characterization. Zoro was never the former, and he's stopped being the latter.
In the case of the latter, Zoro used to have an aura of leadership that was only really exceeded by Nami aboard the ship. And I don't mean that first mate bullshit that fans have deluded themselves into thinking, I mean he was clear-headed and had a clear sense of goal. He wasn't like Usopp or Brook who'd comedically do something that would break himself away from the task at hand. Post-skip, that's changed completely. In Dressrosa, when his sword is taken, he puts their covert mission at risk to chase after it and make a scene. And it's not that chasing after his sword was in of itself wrong, it's just that Oda wrote it in a really weird way since it's out of character for Zoro to create and be in a situation that puts the mission in jeopardy like that.
There's also the case in Zou where Zoro said "**** Sanji," and actually expected the crew to abandon Sanji. His speech in Water 7 with Usopp is such a contrast to his speech on Zou where it was fueled by his petty disdain for Sanji than actually logically assessing the situation. Key point, he says they can't go after Sanji because they'd piss of two Emperors if they do. Except Luffy already pissed Big Mom and Kaido off. And he had no problem with Luffy pissing of an entity that's worse than two Emperors, the World Government, to take back a crewmate who was forced to go with them.
Another thing that made Zoro compelling in spite of just being another stoic badass is that his confidence felt earned and deserved. We saw him train and struggle in his fights, so when he was victorious and his strength was displayed, it was downright human. Now, it's just another stoic badass being arrogant. There's nothing special about it, and this only works for end of series type obstacles or antagonists who are on-off. This makes it harder to be actually invested in Zoro because he's just an end of series characterization we're supposed to be following.
I brought up JoJo because Jotaro, another stoic badass, is an example of what Zoro should be. We don't see him training, and he gets stronger out of nowhere whenever he needs to beat someone, but he's still a vastly entertaining character for pretty much the opposite reasons Zoro isn't. His stoicism is often played for laughs to the point that he's almost a parody of such characters. Even though he's ridiculously strong, he doesn't just go into every fight with "I'm gonna pound you easily." A lot of Jotaro's enemies are shown to be weaker than him in a straight fight, but pose a challenge by having tricky abilities that Jotaro has to get around by being extremely clever and smart before he gets to the stomping part. Zoro just skips straight to the stomping, the closest he's come otherwise is Pica. Jotaro has that underlying, supplementary characterization to support his stoicism, Zoro just has stoicism, and that gets lame as hell quick.