Note that there are probably many ways to fix that part in the game, and this is just one. Feel free to come up with your own, use mine, or whatever else. I am in no way a professional... yet.
Make
Mishima
A Phantom Thief
Seriously. He’s unlocked sometime inside Kamoshida’s Palace, perhaps around the end, somehow having followed you up there. Anyway, he a Navigator for Palaces, instead of buffs for the characters, he debuffs the enemies. You can do his confidant up to a point, and then you can’t further it until later, just like Sojiro’s. Futaba’s Palace rolls around and she joins the team. Over time Mishima starts feeling more and more useless, and rightfully so, Futaba’s pretty much doing everything he was doing but better. Eventually, when Okumura’s Palace rolls around and he notices something’s off with the website, but pays it no mind. Before everyone heads there, he leaves the group and heads for Okumura’s Palace by himself, looking to prove himself. He accidentally brings Haru with and decides to give her a (bad) crash course in Phantom Thievery. When they find them, Mishima’s back in his civilian clothes, unable to summon his Persona, and Haru’s only partially powered up, fighting off enemies.
After everyone else saves them, Mishima has an argument with everyone and storms off, mentioning he’ll still run the phan-site but only out of respect. Mona, curious about what’s going with Mishima and his lack of Persona, decides to tell everyone they’ll go to Mementos later to check on a hunch of his, and they continue on, Haru properly introduces herself, and they go on their way, Haru getting her full power, and such. After that, the team makes a pitstop in Mementos and discover, yep, Mishima has developed a Shadow, due to an expanded ego, depression, and a sense of uselessness. It goes the same way it does in his Confidant previously: Guilt tripping him to death. And now you can continue his Confidant. Once you finally finish his Confidant, he rejoins the team again, fully this time, and you discover his Ultimate Persona is dual purpose, he can either be a navigator, or a fighter now. There’s 2 sets of cutscenes from now on with and without him depending on whether he rejoins the team or not.
And when you get to Shido’s Calling Card Cutscene, you see Futaba and Mishima working together, using their strengths to make it the best way possible. Mishima handling video editing and directing, Futaba handling distribution and guard hacking. Mishima finally over his feeling of uselessness because of her.
Course someone is going to have to “Rally the people” when cup god rolls around, but of course our other option is Mishima is immune to being erased as a Phantom Thief because he’s their #1 Fan, and he’s partially the reason Igor is able to save them. Then, when he’s rallying the people, he gains his outfit once more and flies up to the team, the crowd thoroughly rallied enough to generate their own hype.
