To touch on your point about Bill not being the "super mature and diligent leader they like to make him seem," I think what a lot of people forget (or just don't know if they haven't read the book) is that Bill isn't even necessarily a leader because he chose to be or because he has the correct personality for it. He's a leader because the magical forces at play all throughout that summer made him a leader.
If you've ever read the book, you know how much the narration makes it clear that many of the actions the characters take or the thoughts they have aren't necessarily coming from a place of logic or even intuition. And I don't mean to say that the things they do or think aren't logical or intuitive at all, it's just that that is not where they originate from in this scenario. Two immediate examples of this I can think of are the rock fight and the scene with Richie and Bill where they are walking to the clubhouse before all the Losers will end up discussing the smoke-hole ceremony.
With the rock fight, while perhaps a normal, non-magical version of this scene would've had the characters gathering rocks for ammo because they heard a fucking M80 in the distance and were being chased by bullies all summer AKA "hey guys, I just heard a dangerous firework go off in the place where kids go to mess with fireworks, maybe we should ere on the side of caution just in case our bullies (one of which is capable of murder) are the ones who set it off." Instead, Bill just tells the others to gather ammo and the others "stare at him as though he had gone crazy" and then begin gathering rocks too without question. Richie also folds him glasses up and puts them in his shirt and when asked why he did that by Bev, he tells her he doesn't know, and Eddie thinks about how he can sense that something is going to happen. All this is on page 649 (at least of my copy of IT).
Similarly, in the scene where Richie and Bill are walking to the clubhouse, there is an interesting moment in the conversation where Bill briefly mentions Bev. In this conversation, Richie and Bill are discussing the plan to make silver bullets to shoot It and after Richie essentially asks Bill what his new plan is since Bill believes the plan to make silver bullets won't work, Bill says this:
"'I got an i-i-i-idea,' Bill said. 'Simpler. But only if Beh-Beh-Beverly -'
'If Beverly what?"
'Neh-Never mind.'
And Bill would say no more on the subject."
-pg 692
Once again, if you've read the book, you know that Bill's new plan will be to make silver slugs rather than bullets and that Bev will shoot them at It using Bill's slingshot because she has aim so good its weird (and it is WEIRD. I fully subscribe to the theory that the Losers shine, like Danny and Dick from 'The Shining'). However, the part that makes this strange is that, if you have examined the book timeline, Bill has ZERO idea that Bev will have extraordinary aim because they haven't tested it yet. That won't happen until after Eddie breaks his arm, which happens in Chapter 16, while this conversation takes place in Chapter 15.
Both of the scenes, in my opinion, are pretty perfect examples of the Losers just automatically doing, saying or thinking something because the just know it's right, even if they have zero reason to believe that with no thought going into that belief whatsoever.
The same thing goes for Bill being the leader of the Losers Club. He isn't the leader because they explicitly chose him, because he has the right personality for it or because he made himself the leader. He's the leader because he's supposed to be. In the mind of the Losers Club, that's just right, and they don't have a specific reason for it.
Sure, you could argue that Bev and Eddie are both pointed out as viewing Bill almost like a father or older brother figure, which might place him in an authority-like position towards them, but that's never made out as a reason for Bill being a leader. Those might be contributing factors, but then that wouldn't explain why the other four Losers also view Bill without question as a leader. There's an interaction at the beginning of Chapter 13 between Bill and Richie which further proves my point on this:
"Richie had hung his transistor radio over the lowermost branch of the tree he was leaning against. Although they were in the shade, the sun bounced of the surface of the Kenduskeag, onto the radio's chrome facing, and from there into Bill's eyes.
'T-take that th-thing d-d-d-own, Ruh-Ruh-Richie,' Bill said, 'It's gonna buh-blind m-m-me.'
'Sure, Big Bill,' Richie said at once, with not smartmouth at all, and removed the radio from the branch."
-pg. 619
If the fact that SK points out that Richie doesn't smartmouth Bill at all isn't enough of a clue lmao. Richie is known for always running his mouth and that makes this interaction uncharacteristic of his normal behavior. Had any of the other Losers said that, Richie would've had some joke to make.
In any case, this reblog is getting mighty long, so the point I'm trying to make is that Bill is definitely not a "super mature and diligent leader" at all. He's only a leader because that's the role the Turtle (or whatever other magical, godly forces were at play besides It ofc) wanted him to fill. In reality, Bill is an emotionally-repressed, grieving kid learning to navigate his drastically changed home life and is coping by being practically suicidal on his massive bike to gain a moment of blissful ignorance or by chasing after a murderous shape-shifting space alien in the hopes that if he kills it his parents will show him that they still love him and stop looking at him blankly. Could he still have the potential to be a leader without all the magical forces? Absolutely, but that potential is not the reason he is the leader of the Losers Club in the summer of 58'.
The Bill we see in the 2017 movie, while inspired by his book version, is not the same Bill that SK wrote in the book. Book Bill didn't take charge of the situation because he wanted to, he took charge because he had no other choice, which I think is perhaps best shown in this excerpt from the beginning of Chapter 13:
"Their eyes watched him and he wanted to tell them to look somewhere else, what did they think he was, a freak?
But of course he couldn't do that, because all they were doing was waiting for him to tell them what to do now. They had come by dreadful knowledge, and they needed him to tell them what to do with it. Why me! he wanted to shout at them, but of course he knew that, too. It was because, like it or not, he had been tapped for the position. Because he was the ideaman, because he had lost a brother to whatever it was, but most of all because he had become, in some obscure way he would never completely understand, Big Bill."
- pg. 619
