Not saying the North Blue past was a retcon, saying the self-hate was.
Self-hate is the incorrect term for it. Sanji doesn't hate himself, nor does he lack confidence or self-acceptance. His problem is that he does not attach a healthy value to his life, and that's been a consistent throughline since his introduction.
In the Baratie arc, we learned he became strong
for Zeff and was willing to die to repay his so-called debt with his life. He could not see the restaurant was not the old man's pride and joy. He was.
At Arlong Park, he prevented Zoro from jumping in the water to safe Luffy and did it himself.
On Drum Island, he threw Luffy and Nami to safety and nearly died in the process, breaking ribs and fracturing his spine.
On Skypiea, he saved Usopp and took an El Thor for it.
On Thriller Bark, he insisted Kuma take his head to spare Zoro's life.
You get the gist. There is an imbalance in Sanji. Selfishness is not inherently bad, nor selflessness inherently good. A King must balance the two, and Sanji never has, because the only value he attached to his life is what he could do with it for others. First Zeff, then Luffy and his crew.
Sanji can't become a King for anyone but himself.