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I agree with every bit of this and I (somehow) have even more to add. Akechi living is fundamentally not only better for his arc, for Joker's arc, for the theming of the third semester and persona 5 as a whole, but also (and most importantly) for shuake. I'll detail why in this comprehensive essay/hj
For the same reason you said, Akechi went into his plan without believing he could heal or had any semblance of a future. It never mattered to him and was never on his radar. Being forced to forge a future for himself after everything is said and done would be infinitely better for him than dying as a plot device. It also gives Joker the closure he needed, as Akechi was the only one he couldn't save (and arguably in a lot of ways, Joker was the only one who ever even got close to accomplishing). I also add to this saying that Akechi's survival being ambiguous adds to this even more, because instead of tying it into a neat little bow where Joker got the future with Akechi that he wanted and they can heal together, it's more like Joker maybe got a second chance. The possibility of closure is present, and that's more than enough.
Which leads me into the theming of the third semester. The whole point is Maruki doesn't believe that people can heal from trauma and come out better for it. He believes that trauma will always eat away at a person negatively, and erasing it is the only permanent "fix". Just by having the possibility of Akechi's survival, this actually reinforces Ren's choice to reject Maruki. It proves that a happy future for them is still possible. Just like any of the other thieves, Akechi and Ren can both carve the future they want for themselves, even after Akechi never believed he could.
As for shuake, Akechi's survival is arguably the best outcome to represent their bond. The glove was a promise for them to rematch their duel, yes. Which by extension is a promise that they will see each other again. On one hand you have Akechi's death and his glove in Joker's possession as dramatic irony that fate is too cruel to support such a bold promise. But then he shows up in the third semester, therefore facilitating the hope that the glove's symbolism provided. Ren choosing to reject Maruki's reality is him repaying Akechi for continuing to keep the promise for a rematch, basically saying "I choose to respect you because our promise matters to me". Thematically, Akechi's ambiguous survival is the inverse of this. It's the sappy cliche that the glove/promise is set in stone, and not even fate can deprive them of that rematch. The only thing that can do so is the player choosing not to respect their bond. Something something "you know where to find me/I know where to look"
I think one of my problems with the "Akechi dying is better for the themes of P5R and Joker's character growth and mourning" is like... Okay, but what about Akechi? That framing makes Akechi more of an object to Joker's character, IMO, whereas I think that him surviving in the max confidants ending has so much fascinating potential for Akechi having to live with his mistakes and move forward. He went in expecting to die, to have a simple final act of freedom... But I think him being forced to keep living, to face every day one at a time, to find ways to make peace with what he did and live in the world is just infinitely more compelling from a storytelling standpoint. For Akechi, I think it's a better outcome because it's lacking in elegant simplicity. Even if he lives, he was still willing to die for the sake of everyone's freedom. His survival doesn't erase that. It may "lessen" Joker's choice to reject Maruki's reality, but I think it also makes accepting that reality incredibly cruel, when a strong enough bond with him is enough to save his life and make a miracle happen. Mona describes the world itself as cognition. In P5's vanilla ending, he says it was the PT's bonds that allowed him to continue to exist as a cat in the real world. Given the significance of a maxed confidant making him appear in the postcredits, I believe that it is very simple to read his survival being because of that bond. That wish that they both shared, as confirmed by the Royal artbook. If you prefer exploring the grief and mourning, that's completely fair, but I think there are many ways to interpret Royal's themes, and Akechi living in a harsh reality where you can't escape your past but you can heal and do better is still very on point with what P5R is all about. Unlike P3, which centers more heavily on death, P5R simply touches on it as a part of its greater narrative. And even in P3, you can save a certain character from death via player choice and connections. So, yeah. :p