I have thoughts about that last line of Akutagawa's about luxury. And like. Please if someone with a better knowledge of Japanese has something to add, please do.
So Dazai goes: 芥川くん頑張ったね
"Akutagawa-kun, you did well". 頑張る is such a versatile word. The easiest meaning is perseverance and persistence. So like, "good job making it this far". But I feel that Dazai's words have a more broad meaning there.
And then we have Akutagawa going:
...嗚呼 (basically an interjection, "Ah/Oh")
何と美しき音楽 (What beautiful music)
許されるのか (can it be allowed)
このような贅沢が (this luxury?)
The music phrase I feel refers to Dazai's praise. Akutagawa wanted his praise and acknowledgement for so long, and hearing it is beautiful, of course.
Then the next two. This seems like a single sentence to me. Please note that there is literally no pronoun anywhere, it's open to interpretation, this happens a lot of Japanese sentences. That's why I feel more confident translating Japanese into Russian first to get the feel of the sentence flow, because in Russian you can also omit pronouns and subjects and whatnot.
So. That sentence reads to me as Akutagawa asking himself if he can be allowed to having/enjoying the luxury of Dazai's praise. Asking himself if he truly earned it.
Not acceptance, but a drive to continue clawing at the enemy to survive.
I'm really curious to see what the next chapters will bring, in light of that.
Also, Akutagawa and Atsushi have to meet again. They're the core of this manga. It can't just be the end for them, it's too abrupt that way.