Training arc (being a short arc, around 10 chapters) was just an option. Not my favorite of course.
Luffy needed help beating Doflamingo (had to rest 10 minutes), and Zoro took down Doflamingo's right hand man with no struggle. The gap was small between those two. As I suspect it was with Roger and Rayleigh. Days, weeks. All the same. Next to no time and he is 2x stronger. Why? Oda had to give Luffy a power leap, and because Katakuri wouldn't finish off Luffy. I'm tired of these "what if" fights that Luffy is getting. What if he couldn't rest 10 minutes? What if Nami didn't help him? What if Katakuri didn't impel himself or stop to eat food for comic relief? That's 3 losses instead of 3 wins. The worse was with Katakuri, because Luffy was growing very strong over the course of the battle.
Sanji is barely my 2nd favorite. Him and Zoro are always swapping places. But never in a thousand years will I care more about Sanji than I do Luffy. Both Zoro and Sanji should only be a little behind Luffy. Especially Zoro. The reason I want a Sanji moment in this arc is because of three reasons. Jinbei is joining the crew, and while Jinbei will be the 2nd strongest upon joining, he won't grow much as he is already at his prime, or near it. Both Zoro and Sanji will leap him. The other reason is because Zoro will probably get a moment of growth while Luffy and co. make their way to Wano. The third reason is simple really. Sanji needs a W in the New World. His greatest feat wasn't even the cake, it was him taking command of the Sunny with Luffy and Zoro not there, and fighting back against Big Mom's crew first. But his greatest win in battle has come against that Bobbin guy. A pretty weak guy. Yeah I see him fighting Vergo, Doflamingo, and then Daifuku. Stronger and stronger and stronger opponents. But is he winning? No.
Right. Because a "training arc" that took 10 chapters only would do anything to quell the whining about Luffy's strength. It would go from people crying about the present to crying about how it only took Luffy 10 chapters to grow in strength. It was never an option, just a complaint that made no sense and never did.
Oh, please. The gap was not small between Doflamingo and Pica. Please stop. It has become more than clear that there is a distinct difference between the captain and the next in line. It has been shown abundantly over the life of the series. As far as Luffy "needing help" to beat Doflamingo, lets not forget that Luffy absolutely annihilated Doflamingo while in G4. You mentioned "plot armor" lets not forget that G4 ran out the first time just as Luffy was about to land that last hit on Doflamingo. He needed to recover for 10 minutes, he didn't need help to beat Doflamingo. Doflamingo was completely outclassed. Days and weeks are not "all the same", as we've seen how strength progression scales. These "what ifs" aren't as prominent as you try and make them seem.
No, both Zoro and Sanji should not be "just a little behind Luffy. They shouldn't be close. Zoro might be closer at the end than he is now, but Luffy will always significantly stronger. Zoro's goal is to defeat Mihawk, who when it's all said and done might not even be a top 10 fighter. Meanwhile, Luffy will be walking through top 10 fighters by the time he accomplishes his goal. Also, there is no reason to suggest that Sanji will leap Jinbe. This is nothing more than a way for you to scale Sanji to Zoro. Just because Zoro will probably be stronger than Jinbe (which he will), does not mean that Sanji will. Also, no, Sanji "taking control of the Sunny" was not his biggest W. Like it or not, Sanji's biggest contribution in the NW was the cake. To say that he "took control" of the Sunny is a bit of an exaggeration, as they were all working together anyway and it was based on a gag. Sanji is not Zoro. He is not Zoro's equal. He is not on Zoro's level. He's not near as strong as Zoro. Stop treating him like he is.
Katakuri knew this and didn't finish him off. That is what I can't get over. I'm more angry about that than Nami helping him against Cracker, him getting 10 minutes to rest against Doflamingo, and Katakuri stabbing himself. This reminds me of Luffy getting stronger than Crocodile over the course of the arc, with Crocodile refusing to see Luffy die before walking away. Katakuri could of easily of stabbed Luffy the entire time he was trapped, but decided to eat donuts. That is not how I want Luffy climbing the ranks.
No, Katakuri didn't know that until later in the fight. Nami was only necessary because Cracker was a direct counter to cqc fighters like Luffy. It wasn't because he was stronger than Luffy. Also, funny how you're not mad that Cracker did nothing but run away after their initial scrum, up until the point that he got frustrated and attacked Luffy hours later. Same can be about the 10 minutes thing. As I mentioned earlier, he absolutely obliterated Doflamingo while in G4. Doflamingo was completely outclassed. Luffy was literally winding up for the last attack before he ran out of G4, but you're mad that he needed the 10 minutes instead of that?
Katakuri didn't stab Luffy while he was trapped in the mochi because he didn't think he needed to. This is one of the more lazy arguments the OP fandom has tried to bring up for no other reason than they're pissed because they were wrong about Katakuri and they knew it at the beginning. Katakuri had trapped Luffy and was suffocating him, he had no reason to stab him because he had no reason to think that Luffy would break out of his awakening. It really isn't that hard to grasp. Likewise Katakuri stabbing himself wasn't a terrible thing either. He finally respected Luffy and wanted to fight him on his own without interference. He was making it an equal fight for someone he had just started to see as an equal.