So my issue is that this has almost zero level of maturity. It's explicitly a seinen. That means it's marketed towards male readers who can read a decent amount of kanji, hence indicative of an older demographic. And yet, the behaviors and such all come across like a light novel shounen story, where every single person has this caricature of a personality, without any real depth.
Apart from that, this reminds me of another series. I forget the name and could probably look it up, but it's about a highschool boy who goes back in time and forms a band with a girl who has a terminal illness. That one was decently more serious in tone than this.