@Greg:
I think there's plenty to explore with him, it's justa question of whether it happens here or not.
Oda could pull a Wapol-like flashback during the final assault but I dunno if he'd be satisfied pulling the same narrative stunt twice. (Yes, most arcs are the same structure, flow, and character pools but that's different from how he knocks down his pieces)
Rather than asking what Oda 'must' do, it's often helpful to consider what would benefit his momentum. Sometimes he'll break form, but usually he tries to protect it at all costs during the climax. Honestly, I dunno if he would've pulled that Yamato and Ace bit if it wasn't chp 1000.
I like how Sanji's flashback was in and out, but flowed well. It didn't feel like most flashbacks. I consider his and BM's flashbacks, the major 2 of that arc. Kata and Pedro's mini flashbacks just don't compare to those as that crucial to the arc.
For Zou, it would be Jack's initial attack and Sanji leaving. They felt like 1 flashback, but 2 at the same time. It's Oda's most creative arc up to that point when it comes to structure for me. Being able to stuff so much in an arc, but it doesn't feel rushed or crowded, is astounding. It was maneuvered well as they visit Pekoms and the leaders. It was great setup for a top 5 moment in the series, "Raizo is safe". It's the only post timeskip moment for me that is in the top 5. It's a difficult task to get in there anyway.
While WCI arc has no top 5 moment for me, it has entered my top 5 arcs. While Luffy was never in question as the protagonist, Sanji's narrative didn't feel smothered by Luffy. Even with dragging Luffy vs Kata, I got enough of the cake baking to feel like the Chiffon-Puddin-Sanji alliance could have their mission ruined at many points. He was able to fit in MVP Brook moments, Jinbe joining, Chopper's Brulee express, Nami's homies subplot, Pedro's "last stand", introduction of a lesser known Yonkou crew, while prioritizing Sanji's story.