I don't see much the animated medium could add to this, TBH. To me at least, this is a lot more suitable for a live action adaptation, preferably a movie, given how short a lot of the chapters are. You could probably make a very good 1h45-ish movie out of this story.
As long as it's not like one of those anime adaptations with the excessive close ups and where everything looks too clean. It's gotta have a grungy, realistic feel to it, like a Linklater or Fincher film (although obviously less dramatic in the latter sense), or something like Only Lovers Left Alive or Booksmart. Actually, I'd even take Alice Wu, especially since she does sapphic films well, and the Half of It wasn't as clean as most of the other teen Netflix fair.
But then I'm thinking, it definitely can't be a one on one adaptation, because either the halfway point is the cultural festival reveal or that's the end of the film, and if it's the end then you'd probably want to put a kiss there. If it's the halfway, then the end could be Mitsuki writing her own song and sharing it with Aya.
...I got too into this.