I think the end notes are very helpful as they act to fill in all the gaps.
At this point, It's pretty clear that the manga so far has been more of a sampler of the novel. I mean, this chapter skipped around through years of Nanasaki's life after his break up with Takuma.
It would have been interesting to see a couple chapters dedicated to Hiroki and Nanasaki's crazy relationship. But maybe with how frowned upon drug use is in Japanese society; the mangaka didn't want to show a gay man abusing drugs? I imagine many Japanese readers who aren't aware how many gay men abuse drugs, could be surprised by that. The manga has certainly left out some more of the darker parts of the novel. I'd be curious what the mangaka would have to say in an interview or something, if asked how they went about adapting the manga from the novel.
On one hand, I wish the manga would have adapted more from the novel to get a fuller picture of the whole story. But this is me looking at this through a Westerner's lens. I think the way the manga adaption has worked so far might be more digestible for Japanese audiences. Maybe, idk. The manga would need to be several volumes if it full adapted the novel, I'm sure.
Still, it's been an enjoyable read. I would be so happy if this ever go a TV/movie anime adaptation. Or a live action movie. With that kind of format, I could see more of the novel being adapted.