Let's be real
Sanji will power portrayal hasn't been as consistent as Luffy and zoro.
Sanji hardly got any CoC hint dropped like in case of zoro and luffy.
And, oda had plenty of time to do that - there was whole arc about sanji in form of WCi and then in wano both luffy and zoro trained their haki....but sanji didn't
Luffy and Zoro weren't literally beat and imprisoned into submission in their childhood age!
---Luffy got beat by Ace and Sabo, his superiors. But then they picked him up and laughed with him, building him up, supporting each other.
---Luffy got a mentor during the 2 years, teaching to be stronger and supported him.
---Zoro got beat by Kuina, his superior, but then they laughed it off, and supported each other.
---Zoro got a mentor during the 2 years, teaching and supporting him to be stronger.
---Sanji got beat by his siblings, his at the time superiors and outside a few bandages and a mother who died, got a cell, and mentally as well as emotionally abused. No long support.
---Sanji got no mentor, no support that didn't want to change him, and had to figure everything out himself.
Sanji is last to awaken because he has more shit to deal with at the most delicate years of his life, and has had to figure everything about fighting out himself after Zeff.