
Why did he felt upset when he thought he was about to completely transform?

I think it is because he wants to keep him by his side always and keeping protecting the MC on his fragile phase, so Aki is enjoying this feeling. Once Yo becomes complete vampire he will gain more freedom and be stronger meaning that he will not be so fragile anymore and may not be dependent on him fully.
I see why he would be anxious about Yo finishing his transition, but he will learn that their bond will not break and become stronger ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
I may be completely wrong here but this reasoning make me feel better ٩(๑❛ᴗ❛๑)۶

It was 16 chapters long but it felt like reading a 70+ chapters worth of chapters that gave me a good pacing and interesting characters and plot. Overall, it did gave justice to that 9.0+ ratings. However, the ending just made me feel conflicted. I don't want to make my own endings because at the end of the day, it won't be a canon.
I don't want to have a very vague ending which will bug me every night if someone died or did they became together or shit happens and they both got isekaid. No one will ever know what really happened next. This is why I hate open ending.
But not to say this is bad because of the ending, I just felt betrayed investing my time and all I've got is a vague ending. Although it is the essence of the open ending, which to let the readers theorize and make an ending of their own which appropriate for what they want, but it is also the thing I hate about it.
Nevertheless, kudos for making a story worth remembering in just a few chapters Would love to read more of your future works.
He knows he can.