Then oda should have focused on this.
Oda should have stated that Sanji can't use coc because he is not accepting himself which is VINSMOKE.
He literally asked zoro to kill him in wano if his personality changes to that of his brothers whom were VINSMOKE
HE LITERALLY TOLD LAW HE HATES THE FACT THAT THE INVISIBILITY THAT HE ALWAYS WANTED WAS OBTAINED THROUGH VINSMOKE TECHNOLOGY.
HE DESTROYED HIS WON RAID SUIT BECAUSE HE WAS NOT ACCEPTING OFHIS VINSMOKE LINEAGE.
The same sanji interpreted him loving himself as riding himself of everything vinsmoke.
Oda should have focused on sanji coming to terms with this and understand that just because he is a vinsmoke doesn't mean he is like the rest of his family. Focus on the sacrifice of his mother. Focus on the fact that his mom's.love is why he can't attack women.
Have sanji come to terms with this and accept himself and then get coc
It IS focusing on it.
Because of his trauma as a child just like in real life, he now courts death. Courting death is not the same as being suicidal or neglectful. It is a SUBCONSCIOUS action to risk yourself, and keep taking more and more dangerous jobs or goals because you don't care what happens to you. It's not actively trying to die or just quitting and giving up like apathy.
Let's take another character as an example. I've compared Sanji to Flynn Rider from Tangled before, and Oda loves Disney so it works.
In Tangled, Flynn Rider reveals himself to be an abandoned and unwanted orphan named Eugene. As Flynn Rider he would actively escape from jail and run to not die. He did not want to die. YET, he took riskier and risker thief jobs until he stole the crown which was high treason and was nearly executed. He's also arrogant and conceited as Flynn.
But when Rapunzel tells him she likes Eugene better than Flynn Rider. He tells her "thank you, you'd be THE FIRST," the man didn't like himself even, despite the conceit and self preservation. Because he was tired of that life subconsciously and traumatized from abandonment.
Sanji is EQUALLY traumatized from abandonment. He does not think he should be loved because his childhood told him he does not deserve it. He's not trying to die, and doesn't notice the risks he takes, because he's just trying to save others and finish the goal or risk. Luffy was his "I like Sanji as he is."
ANOTHER character comparison. Watch Gurren Lagann. Simon doesn't believe in himself but still tries to live. It takes Kamina convincing him by saying believe in him (Kamina) who believes in Simon. He couldn't believe in himself so he believes in those who believes in him. And it isn't until Kamina dies that he can believe in Simon who believes in himself.
Sanji seeing himself as a Strawhat no longer needs to believe in Luffy believing in him. He can now believe in Sanji who believes in himself