I don't see where you saw "sudden" change in a movie that had characters constantly evolving throughout its 2 hours.
Max slowly went from a lonely psycho to someone who eventually started caring about those people around him. Nux went from being manipulated into thinking how Immortan Joe wanted him (and everyone else) to, to actually realizing how stuff worked. The girls escaping with Furiosa clearly started to grow mentally stronger through the harshness of the road and situations they were put in, and Furiosa, well... I don't remember how she changed a lot, but she did go through several things in the movie and her original goal of escaping from town and going as far away as possible in search of an oasis was inverted with Max's advice.
It was one of the most clear examples of characters changing and evolving as I've seen in a movie.
Nux - years of brainwashing is erased within a minute. Literally a minute from talking to the redheaded woman. And you misremember his motivations - he didn't realise anything. He thought he disappointed Joe so he started quivering in the truck and then saw the light. And everyone trusted him.
Max - he doesn't even know these people and yet started caring within a day of meeting them? Why? Because a ghost told him to help them. That ghost thing was so silly. This is why Mel Gibson's Max was so much cooler (based on Max Max 2). He didn't care and was true to his character from start to finish.
As for the wives, they were background characters with no development arc. Growing mentally stronger - really reaching there.
nux is dying through out the whole movie, he hast 2 tumors on his back and without the blood from max he wouldn't even be part of the crew
so you have person who is weak and near death and very easy to manipulate, who thinks that he just missed the great opporunity to die in glory 3 times in a row, and this person meets one of the best looking and kind woman he could ever imagine , almost angle like to a dying man, who tells him that maybe it isn't is destiny to die while killing other people
and saying that max cared about them is a bit streched I would say, in the first part he had no choice , he had to trust them and in the end he had nothing left to lose as btw in mad max 2. I remember max helping those people because his car was wrecked and he had nothing to lose.
also mels leathel weapon madness is a lot different to the one max should portray. max is an lonely man in the wilderness an animal , feral like , that's why he doesn't speak and only grunts