Spoilers for Steins;Gate ahead!
As far as I'm aware, most deeper discussions of Steins;Gate revolve around one of two of its more obvious central aspects. On one hand its time travel mechanics tend to get picked apart a lot, with arguments about whether they make sense, if Rintarou basically destroying whole timelines renders the plot meaningless, and so on. On the other hand its theme of the dangers of humans playing god gets brought up a lot, pointing to how you cannot create an outcome where everyone is happy. While both of these things are among what makes S;G special, I think they are only part of its larger theme of fate and how we as humans learn to deal with it as we grow up.
Did you notice how there is no actual main villain in Steins;Gate? The Committee? The threat it poses always remains somewhat abstract. Mayuri dies regardless of whether they intervene or not. Even once they get a face in the form of Moeka and Mr. Tennouji, they turn out not to be some super-villains but an emotionally vulnerable woman tricked into doing bad things and a single-father trying to make ends meet for his daughter. Thus there is no real sense of victory in beating them, there are just two more people to feel sorry about getting wound up in the larger scheme of things. Also, once Rintarou beats the Committee, they immediately get replaced by a new menace, namely the threat of World War III. Both these threats are, on a metaphorical level, manifestations of the greater hardships life has in store for you. You can never achieve total victory in life, there will always be threats beyond your control, and the only thing you can do is try to find the best trade-off for yourself and everyone else. But more often than not there is going to be someone who gets hurt by these decisions (this point actually gets brought up rather often in discussions about the “Changing your Past” theme, but I think this also plays into my argument, so I thought I'd mention it here).
Then what about Doctor Nakabachi? He also is just a clog in the machine. He doesn't have some great agenda or even the ability to foresee the consequences of his actions. He is just some scientist with an ego hurt so deeply he would even murder his own daughter if it meant he could get recognized by his peers. Consequently the final showdown isn't about Rintarou beating him in a fight (which would have been easy, considering Rintarou is probably physically more capable and having the advantage of the element of surprise), but about tricking fate.
I'll come back to both Nakabachi and the true ending later. First I want to talk about how the character arcs in S;G tie into its overarching theme of learning to grow up in the face of calamity. All side heroines who send messages to the past have somewhat parallel arcs (except maybe Moeka, who I already talked about). They revolve around them learning to come to terms with some great misfortune, usually after being shown what life would have been like without it ever befalling them. The story even shows how they live happier lives after accepting their fates. Suzuha has to give up on her time with the lab members or the prospect of ever finding her father, but in turn she achieves her goal of securing the IBN 5100 and lives a happy adult life instead of losing her memories and committing suicide once she remembers her failure. Faris losing her father turns her from a princess waiting to be saved by a white knight into a responsible adult who basically rebuilds a whole part of Tokyo the way she wants. Luka learns her happiness is not tied to her physical sex and that her friends are more important than what her genitals look like (yeah, S;G doesn't handle her character all that well). Their setbacks actually make them grow as human beings. One important aspect about this growth is that they don't just keep part of their inner child intact, it also propels said growth. Suzuha sees her younger self in the adolescent Mr. Tennouji when she takes him in. Faris keeps her love for otaku culture and uses it to transform Akihabara. And in Luka's case, her swordfight roleplay with Rintarou gives her the power to carry on.
Which brings us to Rintarou's character arc. At the beginning of the story, he is basically still a child refusing to grow up. His childish side manifesting as a chuuni alter ego, the mad scientist Hououin Kyouma, seems fitting, seeing how chuunibyou translates to “eighth-grader-syndrome”. Hououin Kyouma is self-absorbed, stupid, careless, and in his own way pretty naive. In the first half, Rintarou is scared of what it means to be an adult, and whenever he feels insecure because of this, he delegates control to his alter ego. Then, when Mayuri dies, he is forced to acknowledge how useless this approach is once confronted with real calamity, but doesn't know what to do instead, so he tries to just turn things back to the way they were before, turning to Kurisu, the most adult and cool-headed of the characters, for help most of the time. The realization that there is no going back as it would mean letting Kurisu die forces him to finally accept the reality of having to become an adult. He sees it as his responsibility to try to save Kurisu, but fails. He only succeeds once he embraces Hououin Kyouma again. This time though, Hououin Kyouma isn't his shield for whenever he doesn't want to confront his anxieties, but rather the spark of positivity and creativity that helps him overcome the seemingly insurmountable adversity in front of him. I guess the name Houou(Phoenix)-in Kyouma (unspeakable truth) becomes pretty self-explanatory foreshadowing once you look at it this way. From this point of view, it also makes total sense that Rintarou's final showdown is against Doctor Nakabachi, who is also a mad scientist, but whose joy for his fringe science got turned into mediocrity through bitterness and pettiness, and is thus the antithesis to the reborn Hououin Kyouma.
Mayuri and Kurisu as characters are also built around the theme of growing up. Mayuri is basically childlike naivete turned to flesh and a symbol for Rintarou's childhood days. Thus his attempt to save her is an attempt to recreate their innocent past. Him distancing himself further from her the longer his journey to save her takes is also a signifier for how this goal is getting further away from him. Her slapping him once he fails to save Kurisu is the culmination of this, showing that there is no going back to the carefree days back at the lab (I still don't like how she gets fridged and turned into a macguffin simultaneously, but whatever). As for Kurisu, her status as a child prodigy caused her to only be around adults from a very young age, forcing her to grow up very quickly and suppress her more childish personality traits. Thus the general carefree atmosphere of the lab draws her in and over the course of the VN she learns to feel more comfortable with her more youthful character traits.
The true ending also makes a little more sense from this angle than with the “don't play god” interpretation. The latter telling you there are no objectively perfect choices and playing with fate tends to make things worse rather than better gets rejected by the true ending as Rintarou gets his total victory by finding a loophole in the rules of the universe and basically cheats fate. But if you look at it as a story about embracing your inner child, it makes some sense. “Of course you can't escape fate” and “there are no perfect endings” is the way a grown-up without imagination thinks. But who can prove them wrong if not Hououin Kyouma, the ultimate adolescent?