Something I adore about Akaza's backstory is how Koyuki is treated, not just by him but by the author.
He treats her as a human being when caring for her, never sees her as a liability, and gives her hope that things can turn out better for her. She was sick but her sickness wasnt terminal like Ruka's.
When she felt bad about him missing out on the fireworks because he was caring for her, he told her they could go next year, and if not next year then the next year. He didnt mind doing all this for Koyuki. Sacrificing his time for her wasnt transactional for him which is why the next part was so important and a great change of pace.
It wasnt Akaza that asked Koyuki to marry him when she became well enough, it was her and it changed the whole dynamic in their relationship in such a positive way. If Akaza had asked her, even if the reason was innocent, it would have felt like something Koyuki had to do because he spent so much time caring for him. Even if she loved him just as much back, the narrative would have given off the idea that this is how a kind deed will be rewarded. It puts the decision making in Akaza's hands.
Instead, with Koyuki choosing and asking him, she is the one that gets to make that decision for herself. She was given that independence and choice to ask for his hand in marriage. His good deed was rewarded not because he asked for it, but because he proved himself to be a wonderful person suitable to be Koyuki's husband. A decision she made. Nobody else.
