I always thought one if the reason akechi is so distant from the phantom thieves is because the feeling of being accepted and around people who care for him is just so foreign to him he rejects it
yeah tbh thats what ive always seen/thought it to be the case. and from his perspective, theyre enemies. its what ive intepreted the scene post-akechi boss fight in shido palace, when the pthieves were talking down akechi and urging him to fight alongside them. and although not easy and w/o injury, it would be possible to cross to the thieves' side even when the cognition arrived.
i think it also stems from akechi's unhealthy/skewed view and experience on relationships (that isnt w sae and joker <-though maybe a special case) since, after his mother died, he was surrounded by adults who only wanted to exploit him. he also had to craft a palpable mask to appease to the larger masses, and as a result of that, he never thought that anyone could care about him on a deeper level and as his true self.
i think above all, the pthieves' honesty and genuineness scared him a lot. that added with the guilt of being responsible for so many deaths (esp the parents of two of the thieves), i dont think he could ever consciously accept the olive branch that the thieves are offering