Oh, well, then we can agree to disagree. The splitting of the group is in my view a huge mistake in judgement from the writers.
It happens. If we all agreed on everything all the time, there wouldn't be any reason for a conversation. My personal view is that you are letting too much of the book (which we both love) intrude upon your judgement of these events in the film. I think don't think the Losers in the film had yet forged into the kind of group that can withstand fighting monsters. I think it was only natural for that group to break apart. I think subsequent viewings will change your mind.
In regards to the shallowness of the characters, I agree that some were better represented than others. I think the film needed to be longer. To tell the truth and shame the devil, there should have been three films, two with the kid Losers, and one with the adults. The first film should have gone up through the rock fight and the joining of all seven and their subsequent group encounter with the slideshow. This would have given more time for each Loser and to build more bonds. The second film should have been their growing battles with Bowers and the Clown culminating with their battle in the sewers. Two films of good length would have allowed the kind of depth in all the Losers that have been perfect. Still, I think they did well with the time they got. I think if they had known how well the film would do, the studio would have gone for a trilogy out of the gate. They didn't know.