Rebecca and her entire character arc were trash.
That aside, I think a big glaring flaw with one piece and many other shonen series is that they love doing all sorts of shit with regards to female tropes and portrayal that just piles onto eachother.
One example outside of one piece: I've always found that Fairy Tale technically does a better job at giving female characters action-based fights that are more downright physical and significant compared to One Piece.
On the flipside, FT loves sexualizing female characters at every possible moment. fighting? sexualize it. dying? sexualize it. getting tortured? sexualize it. dramatic emotional moment? sexualize it.
A series can simultaneously do something good (lots of strong or important female characters) while also being about as bad at portraying characters in a decent humanizing way. (by trying to get sexy "fanservice" shots in even when a girl is dying from being impaled, for example).
One Piece is kind of an odd case because due to sheer length of the series & oda's style and aesthetics it has a simpson-like ability to have had almost any possible possible character type at some point.
The result of this is that "oda's women look the same" & "oda's women are some of the most diverse in manga" are both true depending on where you place your focus.
While I think OP has some really fun and interesting designs for female characters, like otohime, perona (face, at least), boa's sister who looks like hey arnold & various less important characters; it's also obvious that when Oda feels like drawing "sexah woman" he often resorts to basically drawing nami clones.
Nami, Robin, Tashigi, Alvida, Kalifa, Perona (her body at least), Boa Hancock, Rebecca, Miss doublefinger etc etc all fit a pretty blatant template.
While some people may actively support creepy fanservice & waifu bait designs, I do think when somebody's reaction to this is negative it'll add a bunch of background noise while reading OP. Which brings me to the current discussion:
In isolation Sanji's chivalry is kinda odd and regressive, but it can also be seen as just a character trait; "oh sanji's weird with woman & he's a hypocrite cause he's also a perv".
Which is ... eh. Not the best trait, not the worst ... but simultaneously it's not like Sanji's the only character with weird woman stuff.
The weird ass Zoro-Monet stuff aside, one thing that annoyed the hell out of me personally is that in the new world we basically had:
Sanji, Kinemon, Brook & that little brat all as pervs with creepy ass jokes sexualizing nami and robin. (bonus points for throwing in fanservice that implies the little molester kid took baths with the female characters that were clearly sexualized in the art, basically making it seem like the visuals suggested "these pervs have a point";
Even if a single character having weird views on women can be seen as just a trait if isolated, when half the crew is perving over girls who are sexualized to the readers & even the non-pervs randomly get "lol women" traits in combat ... it sure feels like oda loves him some ...weird gendered tropes. (not to mention the whole nami/robin blankfacing franky thing)
/fake edit: I'm not even gonna go into the weird ass franky hard-boiled fight where he just randomly kisses that villain woman.
Even if you don't like the word sexist for it, there's a clear gender-esentialist streak to oda's humor and writing where a lot of jokes seem to be derived from "boys will be boys" & "girls will be girls"-type of writing.
Now does this mean Oda's writing is the worst ever and his female portrayals are completely irredeemable and without positive points? I don't think so.
Despite having more straight up physical fighters fairy tale manages to throw in fanservice at so many inappropriate moments that it manages to beat OP in my eyes by just being so damn in your face with the creepy objectification.
In my view OP's biggest issue is that unlike FT's one clear issue, there's just all these little grievances that can be explained away individually, that do add up over time.
On paper Nico Robin's my favorite strawhat & I've been wanting some more cool and interesting moments for her. Especially as she's basically never had a good 1 vs 1 beyond that turd priest in Skypiea. So what do we get in dressrosa?
... a bunch of fanservice jokes here and there (bathing with the creepy kid, getting tied up by tontatta) and when it seems like she'll finally get her big 1 on 1 ... she's just there to protect Rebecca. (who's fucking shit btw).
I could probably name a thousand moments like that, which are "logical" within the narrative. (yeah sure, diamante should be beaten by rebecca or kyros) but which just add to the irritations I have with OP and gender stuff.
Like any time a really major or powerful new female character in OP shows up my gut reaction is: "I bet she has a devil fruit power that can instantly disable opponents without actually needing to fight that much".
Why do I think that? Cause Kalifa, Perona, Hancock, Giola, Sugar, Mirror-Lady, a bunch of movie characters all follow that trope.
I notice that a lot of the arguments here are fairly "I found this moment to feel sorta sexist" with the rebuttle being "yeah but it made sense" or "yeah but this char is cool and strong"; but if stuff like that bothers you at all, OP has a ton of it.
Anyway sorry for rambling.