Inverted Dazatsu thoughts:
I feel like their relationship in that setting would be one of frustration, but also attachment. VERY unbalanced.
Dazai would feel guilty for dragging (directly or indirectly) Atsushi into the ADA, which is corrupt, and feel like he needs to protect him in some way. But Atsushi, on the other hand, is indifferent to Dazai's sense of guilt since he doesn't feel bad in this type of environment and wouldn't necessarily feel the same about protecting Dazai, not in the same way, at least (he would step in if needed).
Unlike Dazai, who would try to push Atsushi to do the right thing, Atsushi isn't really interested in morality, making the tactician frustrated. Dazai could push Atsushi to reconsider his actions, but Atsushi would usually respond by mocking him or outright ignoring his advice. Dazai might often question if it's even worth trying to save someone who just doesn't care.
That, of course, would settle a sense of hopelessness in him, knowing that he's been forced into a system he hates and now he's trying to protect someone who embraces the system. There would be moments when Dazai might even consider whether Atsushi is lost beyond saving, but at the same time, Dazai would be unwilling to give up on him completely.
Atsushi would definitely be strangely attached to him, even if he recognizes Dazai's "idealism" (since that is what Atsushi views Dazai's morality as) and sees it as weakness. He might even be a bit amused by Dazai's attempts to save him, even when he doesn't need it. Probably, he wouldn't admit it, but it did kind of make him feel warm inside, something he has never felt.
However, his indifference wouldn't mean that he doesn't care about Dazai at all. He has a more sardonic, almost detached form of affection. Atsushi might not actively try to save Dazai from the ADA’s control, but he also might not let anything happen to him.
Their more romantic tension would be more of a push-and-pull kind of thing, because how do you even navigate a romantic relationship when one of them doesn't even want to be there in the first place, in an environment like that? Dazai might try to show genuine care for Atsushi, but Atsushi might reject it. However, deep down, Dazai might be the only person Atsushi can tolerate on some level, even if he’s not willing to admit it.
Atsushi, in his own way, might appreciate Dazai’s stubbornness and flawed compassion, finding it oddly endearing, but would still keep him at arm’s length. His emotional detachment makes this a painfully slow burn with complicated and lots of emotional distance (on both sides as well, Dazai isn’t going to just take shit from Atsushi).