I'm sorry, but underdeveloped? One Piece got way more "main cast development" than pretty much any other shonen out there for years. Nearly 14 years of growth Oda did in the main characters before the timeskip.
Realize that with post-timeskip, the Strawhats understand, respect and love each other now. There shouldn't be anymore internal conflict, they grew past that with plenty of arcs pre-timeskip.
Its already there, but you just want more of it. That's the difference. And it's okay to want more. I get why people complain about the changes post-timeskip to more world building type arc. But can we stop acting like the Strawhats are underdeveloped? Many of the characters have continuously received additional development over the years since the timeskip:
- Nami at FI
- Chopper at Punk Hazard, though he became a sacrificial lamb after
- Usopp at Dressrosa PLENTY
- Sanji and Brook at WCI
- Now Zoro at Wano (and will likely continue to have development)
Just remember that as this manga finishes this arc and heads towards other arcs or characters, like Vegapunks/Kuma, Elbaf, Lodestar/Laugh Tale etc, stories will become more personal again because they heavily revolve around some strawhats, namely Franky, Robin and Usopp.
Oda has been building Wano plotline for nearly a decade, and yes while "personal" development of his main characters has taken a backseat, it allowed him to go more epic with arcs and dealing with Yonko.
Just be patient.