I just kinda say stuff

Fukuzawa = Atsushi, Mori = Akutagawa

Fukuzawa and Atsushi both strive to help people despite the fact that they are both capable of harm and destruction (Fukuzawa being ex-assassin and Atsushi with how his Ability used to be completely out of his control); while Mori and Akutagawa are both abusers but at their cores they want to be able to care for people (Mori being an ex-doctor and Akutagawa being protective over people he deems worthy of his respect).

Dazai and Chuuya technically fit that mold as well, but what differentiates them is that they both have tragedy attached to them in a way that the other Soukoku generations don't. It's implied in SB that Chuuya (like Kouyou) didn't WANT to stay in the Port Mafia, but he eventually accepted that he wouldn't be able to leave and THAT is why he's grown so dedicated to them - but Dazai joined the Mafia because he thought that immersing himself in violence and death might give him a better appreciation for life, and only left because Oda suggested it. Dazai and Chuuya are in the wrong places - the tragedy comes from the fact that they're each trapped in a world the other would prefer to be in, and it doesn't seem like that will ever change. (Not that Dazai should've stayed in the Mafia or that he's incapable of change, of course, but my point here is Dazai chose to join the Mafia while Chuuya didn't).

Moreover, Fukuzawa and Mori's relationship is a rivalry in the sense that they are on opposing sides, but we get to see some of what they were like with each other before the Detective Agency came into being - and it's much closer to something like friendship than them being enemies. They clearly know and respect each other, even if they don't see eye-to-eye on everything - and even at present, it's not a stretch to say there's some level of mutual trust between them, even just in the sense that they can safely predict each other's moves. At this point in the main story, it's not unreasonable to make the claim that Akutagawa and Atsushi's relationship is also trending toward something like that.

Dazai and Chuuya's relationship is not like that.

Dazai and Chuuya are not friends. They work well together, and they obviously trust each other, but however you look at it, their bond is not friendship. Whereas it could be conceivable that Akutagawa and Atsushi / Mori and Fukuzawa respectively would get to a point where they might admit that they're fond of each other despite the conflict between them, Dazai and Chuuya would never admit anything of the sort. There is genuine dislike between them due to their differences, and despite their unshakeable trust in each other, there seems to be nothing that would ever bridge that gap; even Dazai's return after being presumed dead by Chuuya for four years wasn't enough to make them even pretend to be glad to see each other again.

Also, there's a feeling of maturity to (current) Akutagawa and Atsushi's dynamic that simply is not there with Dazai and Chuuya. The reason Akutagawa and Atsushi work together as a team so well is that thematically the point of their relationship is that they help each other learn and grow as people, while thematically the point of Dazai and Chuuya's relationship is that they give each other the chance to act like the children they never got to be.

Mori and Fukuzawa shaped the Port Mafia and the Armed Detective Agency. Akutagawa and Atsushi are learning to shape their own lives.