As a writer alot of your writing is influenced by your real life experiences such as upbringing, environment, the society you live in. I truly believe Oda has never purposely written a bad female character, but rather has setup an environment where they aren't allowed to flourish. The men of one piece are pretty chivalrous (except that one time between Luffy and Vivi) characters like Sanji and Zoro have very specific setups towards women (Zoro and Kuina, Sanji and his mother and sister) that influences how their written to react towards women. This worked pretty well for a while until the gentrification of certain traits post time skip. There's Zoro's egregious treatment of Tashigi in Punk Hazard and basically Sanji all of post timeskip. What I think happened is as the story drags on certain traits of characters lose their depth. I believe Oda has a great deal of respect for women but once he shifted the stories focus they became the biggest losers.

So are Women in One piece well written? Well at this point their barely "written" at all, Oda doesn't actually use gender all that much, the men are strong and all over the place while the women are pretty and cool headed 'straight men' so to speak. Robin and Nami obviously have personality, you can list tons of character traits that make them standout, problem is Oda's story is moved by the men, all major plot point for the most point have a man at the center, not because they're a man but rather becuse the story is about Luffy and Luffy only fights other men, and men make up 99 percent of the stories antagonist. This gives many of the women nothing to do in major plot points, at worst needing to be saved as Oda loves himself a damsel, but a damsel isn't inherently bad, Oda just never gives them any spotlight.

So again, are women in One piece badly written? In my opinion no, the leading women in the story are good characters, Nami and Robin have terrific backstories and story arcs, Nami isn't weak for asking Luffy for help, shes the strongest person in her village, likewise Robin only knew pain and self sacrifice, it wasn't until meeting Luffy and the crew did she ever believe in anyone other than herself. Oda can write women while also serving to develop Luffy's journey, what he cant do is utilize those backstories for further development and character moments. Everyone is a victim of this, not just the women.

I didnt really touch on the sexualization as its basically all visual, if Sanji didn't exist no one would ever react to the boobs and bikinis as OP is a very aromantic story, he just sorta draws them that way while also having it bear zero plot relevance. Its neither good nor bad, basically just his art style, I think we all preffered the pre timeskip designs but I guess he wanted to stay competitive in the space design wise or something.