@Zhenja:
The challenge was to get better at cooking.
Luffy trained to get stronger and learn new fighting techniques.
Zoro trained to get better with the swords.
Nami learned more about weather, so she'll be able to navigate properly.
Chopper learned new things about medicine.
Lysop got better at snipping and a little bit brave.
Frankie learned more about technology.
Brook got a better musician i guess and Robin at reading? I don't know.
Sanji got better at cooking.
Everyone of them has a specialty except Luffy and Zoro, those two are fighters nothing else (except heavy eater and drinker :ninja:) they can only get stronger and learn new fighting techniques. The others can fight too but that's not their main purpose.
Now I'm just jumping in, not really in the conversation but want to contribute slightly. Also what I say down below isn't a complete counter argument to the points you were trying to make but it was a good enough post for me to step onto my apple box.
There is a train of thought that has been going around for almost 10 years now that it was basically only Luffy and Zoro who trained to get physically stronger/more capable combatively over the 2 year skip. Each crew mate specializes in one field or another but each of their specialties has combative application. Each one except for Robin and even then I won't say that that is entirely true.
Nami learned more about the science behind weather in general as well as the weather that occurs in lands yet to come and got new tools to weaponize weather. She came back with absurdly powerful techniques and technologies.
Chopper was on an island with a lot of rare plants and natives who were very advanced despite their looks. He learned more medicine and improved his drug the rumble, which basically gave him instant access to what is almost certainly his controlled awakened form. Not only that he found away to go beyond just about every Zoan that has appeared in the series thus far. Before we met Chopper he used his medicinal know how to create the Rumble Ball, a drug that would disrupt the wave lengths/properties of a Zoan user and allow for more forms within a 3 minute limit. During his time alone it's almost another certainty that he developed an even better drug that permanently altered his body structure so he would no longer need a Rumble Ball for any of his transformations aside from Monster Point. Chopper learned medicine in large part to improve his combative ability. Tell me otherwise I dare you! I mean just read Chopper talking to himself about being a real monster for Luffy, he went it with the thought that he was going to come out physically more powerful.
Simplifying the training saying Usopp went to get better at sniping, Nami went to learn more about weather, Brook to get better at music(is that even right?) and Sanji to get better at cooking completely ignores the combative aspects that each of their respective skills bring to the crew and why they stayed behind to train in the first place. To get stronger.
Franky learned and worked on some new technology but what for? To get ready for bigger battles to come. Battles for himself, the Sunny or even other members of the crew.
Sanji was happy to learn of the secret recipes because he knew what it would mean for the crew and he was always happy to battle the 99 masters for his chance to get physically stronger. Both aspects of his 2 years of training revolved around getting physically stronger.
Robin is the only one that's a bit hard to wrap my head around though I'm sure someone smarter than me can work it out. Who can really say how archeology would support her fighting style but at Tequila Wolf Robin resolved(out loud and on panel) to get stronger for her captain and came back with giant limbs and full body clones that more or less allows her to teleport.
Now with all that said it doesn't even matter how much non-combative time the crew had to themselves or with their mentors. Rayleigh left Luffy 6 months early to figure ship out for himself lol and he is still far and away the strongest.
