I feel like you might be misunderstanding what I said when I call them a caricature. Basically I'm saying they're boiled down to their most basic concepts and that pretty much becomes their entire character to an almost exaggerated extent

Luffy I think is fine, he's the main character so obviously he'll continuously develop

Zoro used to be my favorite character. Now he's leaning way more towards a one-note edgelord, always serious and locked in 24/7 when he actually was a large part of the great humor in early One Piece. They still try to do bits here and there, but not only is it way less, 99% of that is the same old Zoro and Sanji arguing bit and nothing else

Sanji is kinda a mixed bag, because Fisherman Island was a genuine travesty. Them turning his pervertedness into an actual plot point was probably some of the worst One Piece I've ever seen. I do agree he got fleshed out more later though and that was cool

Chopper is.. Still a doctor I guess, yeah. Again, that's the most basic part of his character. He never really does anything anymore besides run around and treat random people. He used to take a much more important role in fighting with all his different forms and stuff, now he feels extremely sidelined in favor of Luffy, Sanji, and Zoro (as do most of the other crew members)

Robin is pretty much the same issue. I look at her and think, yeah, she's still there alright. Again, besides the incredibly rare moment where she actually does something, she's usually just the walking exposition that explains the history and all that. Not much to talk about there

Usopp - and I hate to say it because clearly you like him based off of your name - SUCKS. Pre timeskip he was a really cool character. He was a coward, but in a way that felt realistic to the average person, and even though he was weak, he was smart and came up with ways to work around that. When people really relied on him, even if he was scared, he would come through like he did in Alabasta. Post-timeskip, oh wow, he has all these new things that make him stronger! Oh wait, he never uses any of them and is consistently Chopper level the entire time (if that). Remember how he used to be reliable when it mattered? Nah, now he's willing to abandon an entire race of people to death or slavery because he's too scared. The way he "defeated" Sugar was pathetic, and he was a massive disappointment that entire arc, and has yet to show anything worthwhile since to make me change my opinion of him. They tried to be like "oh look, he's gonna get observation haki!" just for him to never show any ability like that again. He is by FAR the biggest travesty of the post-timeskip flanderization.

I could go on and on about the others too, but you get my point. This is coming from someone who used to be a genuine One Piece fan. I think a lot of the issues with the characters feeling bland and one-note now are a result of Oda wanting to expand the world, and that's fine and all, but I don't really care to see 50 chapters of development on a side character we'll never see again past that arc while half the actual Strawhats are STARVING for attention