It's not that bad.
The idea of splitting them was good since the main goal was to rescue Sanji, not to confront BM pirates. BM was not the main problem, it was Kaido since they messed up his Smile business in DR. So it made sense for a reduced group to go and do that swiftly, after all they didn't know about Sanji's past so that seemed something easy to do.
Here the problem lies with the another half of SH like you said, nothing relevant happened with them. A subplot with Zoro lost in Wano, finding Hitetsu, learning about Ryuma, advanced CoA, permanent Haki, etc, would make sense. But nothing happened... The problem with smiles he only learned about it after all people were there already.
Law has one of the best DF and can move freely around, nothing happened again, it would have been a good moment for Law to encounter with Drake and start something from there. They have some fate after all since kids.
Another key character which Oda didnt exploit here is Caribou. Captured even earlier, how he did not open his mouth about Poseidon in exchange for joining Kaido or at least his freedom? And a subplot with the half of these SH trying to prevent him for example.
It felt like you said, Oda splitted the team in half in DR and to compensate the other half he used them for the next arc while ignoring the others. At least in Zou something meaningful happened with that half deciding to rescue the Minks and involve with them and after that the thing with Caesar.
The idea of splitting them was good since the main goal was to rescue Sanji, not to confront BM pirates. BM was not the main problem, it was Kaido since they messed up his Smile business in DR. So it made sense for a reduced group to go and do that swiftly, after all they didn't know about Sanji's past so that seemed something easy to do.
Here the problem lies with the another half of SH like you said, nothing relevant happened with them. A subplot with Zoro lost in Wano, finding Hitetsu, learning about Ryuma, advanced CoA, permanent Haki, etc, would make sense. But nothing happened... The problem with smiles he only learned about it after all people were there already.
Law has one of the best DF and can move freely around, nothing happened again, it would have been a good moment for Law to encounter with Drake and start something from there. They have some fate after all since kids.
Another key character which Oda didnt exploit here is Caribou. Captured even earlier, how he did not open his mouth about Poseidon in exchange for joining Kaido or at least his freedom? And a subplot with the half of these SH trying to prevent him for example.
It felt like you said, Oda splitted the team in half in DR and to compensate the other half he used them for the next arc while ignoring the others. At least in Zou something meaningful happened with that half deciding to rescue the Minks and involve with them and after that the thing with Caesar.