I dont think the problem is Sanji....the problem is Oda. The fact that he couldnt spare Sanji a few panels of showing growth....instead of going from point A to point B is annoying.
Maybe if he showed something that influenced Sanji to get to the mindset. Maybe if he showed Sanji's inner struggle and thoughts on wearing it. Maybe if he showed Sanji struggle instead of just using one base kick on P1. Anything would be enough. But nooooo. All he did was make Sanji promise to never wear it, then 20 chapters later Sanji is magically ok with wearing it. Im not arguing against the possibility that Sanji could show such growth. Im arguing against the lack of context behind the growth that was given. When did Sanji realise that maybe he needs the RS for this. He surely didnt think so before Wano. So when did this growth occur ? Against Kaido ? When he saw the Boro Breath ?
This shit annoys the heck out of me, and Oda has done exactly the same shit with Katakuri. To the point where he made me despise the L v K fight. He forreal be skipping all the important development, for such unneeded stupid flashbacks. Its jarring af.
But Sanji did struggle though, and quite a bit on WC Island.
- The whole reason why he conceded to Lilin's and Germa's wishes in the first place even to the point where he completely rejected the crew was because he didn't think he and the SHs were strong enough to get out of the shithole they were and as such didn't want to endanger their lives and the lives of the hostages the aforementioned had on him, which kinda shows that Sanji and the SHs in general are still quite weak in comparison to a Yonkou.
- Sanji had a well thought out plan for the rescue mission on Cocoa island and was confident he was going to be able to save Luffy with it but again he failed simply because the strength of a Yonkou was beyond his expectations. Sanji didn't even dream that Lilin was going to have multiple Geppo users on her crew and had to be saved by Germa in order to rescue Luffy. Which again emphasize how out of their depth Sanji and the SHs are in comparison to the Yonkou crew.
- Pedro died as a result of the Sanji rescue mission, and Jinbe and his old crew as well as the Germa had to stay behind to hold the BM pirates back in order for the SHs to escape. Again emphasizing the SHs are still very weak in comparison to a Yonkou crew.
All those things I think are enough justification for Sanji to have decided to try on the RS.
Hell I'm sure Sanji experiencing Kaidou's Boro's breath and hearing about Luffy getting destroyed by Kaidou also helped to hasten that decision.