I apologize if I am assuming you are worried about Sanji's trajectory in the manga, but I completely share your feelings. Although not identical in reason, I feel as if Oda dropped the ball with the treatment of Sanji. This is retrospect to the Time-Skip, which I can go in-depth if need be, but Sanji was supposed to experience this growth to prepare him for the new world.
Considering that the Straw Hat Pirates steam rolled Fishman Island and Punk Hazard, and he was snuffed out the story line in Dressrora, Sanji's growth was never truly shown. At least the unleashed potential thus far (same for many Crew Members) so it was disappointing that Sanji was treated rather "fodder" against the Big Mom Pirates. Do not mistake me for wishing he actually triumph over them (although a nice drawn out fight between Sanji and Oven or Daifuku would have be an opportunity).
I am saying that his Vinsmoke Plotline in all honestly is pointless. The Year of Sanji turned out to be the Year of the Yonkou Saga. There is no growth whatsoever with Sanji during that arc. Dissect this perspective. What did we learn about Sanji that actually changed who Sanji was as a character? Absolutely nothing considering he holds the same values and morals instilled by Zeff. He is not a different character after that Story Arc.
What growth of power, skill, or determination did Sanji experience? Whole Cake Island featured Sanji being manipulated and twisted to the point that the "Powerhouse" we have come to known about Sanji was not a factor. This is not me saying he isn't a powerhouse for the crew anymore, simply that he was not able to showcase what makes him such. We learned that Sanji can cook, faster than normal? What does that do to serve the manga or the character honestly?
We know he is an amazing chef. Cooking "faster than normal" is not even a substantial piece of information have. Did his resolves change or strengthen? Nothing indicates he feels any stronger for his crew or family more than what we already knew. Sanji has tried to give his life for Luffy and Zeff before, so willing to do it now is nothing new. The only thing that possibly changed was Sanji's hate for his family. It went from hate to extreme dislike, which means nothing. All in all, Sanji dislikes his family.
So what are the great values coming from Whole Cake Island? We as fans were given a sad backstory for Sanji, which all Straw Hats have, so that allowed us to like the character more. Me? Sanji was already my favorite, but I do appreciate Oda for adding that value. The last value? (Sorry if this is long winded, but I have arrived.) That damn Raid Suit. The peak potential I was referring to earlier? Its sad to think of it like this, but Sanji's peak potential was either going to be boring (I can kick harder and move faster) OR ridiculous (somehow just igniting with flames).
Sanji will definitely get his drawn out fight where we truly learn about his physical prowess, but the character will be flat. Getting stronger and faster is boring, getting stronger and faster on top of new abilities keeps the character fresh. As for my worries? The Raid Suits are colorful ass costumes that just add protection as far as Whole Cake Island revealed. Soooooo. Sanji is stronger, faster, and now more tanky..........
R.I.P Sanji is what it seems like T-T