I missed an earlier opportunity to talk about this, I apologise to whoever it was that brought this point before, I'll answer it here. I think for sure her cuck arc isn't over. What follows is my preliminary interpretation however, I haven't replayed the event and welcome points I may have missed.
Well, prior to the beach event, we've seen how Osako has an issue with their relationship. What we saw in this update is how Wakana does. Wakana comes to terms with the fact that she has never being able to make Osako understand that she's enough, and that if she couldn't until this point, that is very unlikely to change. Wakana would never betray Osako because she does love Osako, and her sense of duty supercedes her sexual urges. That takes a dark turn however if Wakana understands that she is not enough for Osako.
That's what will break up their relationship. When Osako says that to Wakana, the only issue is that Osako can't accept the truth that Wakana is satisfied with her as a partner (maybe not sexually, but Wakana has truly made her peace with it). But when Wakana says that to Osako it's a recognition of reality rather than a refusal of it. Nothing Wakana can do will be enough for Osako, because Osako's issue has to be fixed by Osako herself, it's not up to Wakana. And since Osako can't deal with her issues, it damns their relationship as a whole.
Their walk on the beach was the beginning of the end. From the moment Wakana realised the truth it's all downhill from here. It's no surprise she felt horny by having Akira there, and Akira seems to actively perceive that it was the two of them that should have been together there. It's because if Wakana is the one making Osako's life miserable, and the only option to make her happy is to leave it (since Osako can't fix her insecurities while by her side), then why is Wakana resisting exactly? Doesn't Osako want her to experiment with Akira? Doesn't Wakana heavily care for Akira already and most likely wants to be with him anyway? Isn't her relationship with Osako doomed by her presence as someone that can't give her partner the trust she needs in her feelings?
The foursome, for me, is based on desperation/recognition of defeat. They're going to break up, and it will fall to Wakana to force Osako into a healthier life she doesn't have the courage to reach for in the first place.
