Sry, but I gotta disagree with you there.
I dont think Sanji has "half ass haki" at all. xD
"oh but he didnt get this or that". Story is not over yet. We also have lots of story stretches without any growth for most characters.
FI didnt have anything aside from basic haki for the M3.
Dressrosa Sanji left in the middle. Zoro only got the ryou blackening on his swords and some black blade tease from Mihawk, but still, it's just basic blackening applied to his swords. The Kujas were imbuing their weapons with invisible haki back in pre-TS.
Zou? nothing happens there.
WCI? Zoro isn't there. Luffy gets the FS, but we get a glimpse of Sanji being able to dodge Katakuri's bean, which considering Kuri is a FS user and couldn't tell Sanji's dodge, it says a lot of about Sanji's CoO.
Wano? CoA+Exoskeleton producing IJ (which, by your standard, should be good, considering it's something more unique to Sanji). Zoro got his sucked CoC from Enma. Luffy, being the boring MC that he is, is the one that got ID CoA and "mastered" ACoC usage counsciously.
Also, despite being shown as a gag, Sanji has CONSISTENTLY demonstrated good CoO showings with his "women detector". And another reminder that DJ can burn people internally. It's the FIRST thing Jabura says when he gets hit by it for the first time. So he doesn't need ID CoA and can dodge attacks from FS user.
Egghead? Nothing for nobody.
Elbaph? Zoro's gonna learn how to use ACoC at will, like Luffy did at Wano. Sanji is getting CoC. He possibly (and probably) will also get ACoC, since he also listened to Gaban's talk about ACoC usage. So he SHOULD be aware of the coating thing already.
So, again, this whole "sanji has underdeveloped haki" is more of a falacy than anything, really. When you actually look at the story, You see that Sanji is perfectly fine.