We're receiving way more information than we would've received if the manga did as you suggested. For one, we got a lot more stuff on Yuu, we witnessed him act and react and be something more than an inscrutable object of Hikari's admiration. We've seen him and Yami "fuck like rabbits" as you say, we see how, under what circumstances and motivations, something we wouldn't have gotten if we were just told "they fucked like rabbits". And if that is a "multi-chapter monologue" and "a fuckin text dump", then how exactly are the first 20 chapters different?
This IS the story, it is no less important for the author to tell and for us to see than the events chronologically right after chapter 21.
Do you want it to be a bunch of small flashbacks? Where do you want them to be interspersed? So far I don't see how that would be any better.
it would be better interspersed because it wouldn’t have ground all narrative momentum to a screeching halt at the climax of the story. None of the involved characters can react to this huge lord dump because this was just Yami thinking to herself. The issues need to be discussed so it all needs to be brought back up anyway.
Ideally these could have been peppered in during heart to heart moments between the characters, instead it’s all shoved off on its own. Just as an example there could be a scene where Hikari finally sits down with and asks when they even met and then a small chapter or two flashback while she tells part of the story, then Hikari naturally reacting to it. There could be a scene where Hikari confronts Yuu about what happened and why they kissed and we get a short flashback that Yami had told him. There could be a scene where Yuu confronts Yami and asks why everything fell apart and she’s flashbacks reviewing the parts she told Yuu and adding details from her perspective. During all of this the characters can act and react with eachother and progress the narrative. The issue with a flashback is it can only provide context, it can’t progress a story in progress on its own because everything that happens during it is, by its nature, already done.
The key is don’t drop it all in a huge lump if the characters involved still need to deal with it in the present day. It kills the momentum and shock the author had built up during the first arc. The feeling of “oh my god what’s gonna happen I’m so invested” died at the end of volume one and was replaced with “can we get some actual progress now?”
There’s a reason that stories which make heavy use of flashbacks either keep them short and tight or make them part of a discussion between two characters as a framing device, to avoid narrative redundancy. No one can react to this entire arc because no one but Yami knows anything about it.
She can either A. Not tell anyone ever in which case this should have been inserted before or after the climax so as not to disrupt the flow or B. Tell them in which case it should have been shown when the specifics are relevant.
We don’t need to know that the pair spends days fucking in a hotel to resolve why Yami is missing Yuu, that can be brought up when the topic of hotels is (either Yami and Yuu reflecting on the last time they went to one, Yami thinking about it while at a hotel with Hikari/the other girls/someone mentioning going to a hotel with someone, or Hikari and Yuu dealing with it if/when they go to a hotel or the topic of sex comes up) which would keep the pacing up and continue the evolving narrative. And as you can see even if the author DID want to switch to Yami’s POV they had plenty of ways to dole out the information in smaller, more digestible chunks rather than trying to swallow the whole watermelon, again, IN THE MIDDLE OF THE CLIMAX.