I remember the 90s. Great times… :)

This game has a fun feel. Your design doc seems to have a good handle on what you need to do. I think the choice paralysis on the music would be one of the biggest challenges to overcome for me personally.

I also have the same question as Khaprani, did we have “lofi” in the 90s?

I think this could be fun, I wouldn't sweat the difficulty of the maths portion for yourself, it is going to boil down to summing up how much and in what way the love interest likes the songs, then adding those up and deciding what mood she put the love interest in. Ultimately the part that is going to get sticky is deciding how to key your character in that a certain song will make the love interest feel a certain way, do you do this with clues, does she get some shared interactions where these are in the background, and those go good or bad (an "our song" kind of thing) what drives those numbers and how to communicate them and give the player some choice in how they happen. That's where I'd spend my time.  Maybe circle in on the key sequential moments and start working through those. Also, did we call it lofi in the 90s? Did I miss the boat on that one?

It has an interesting premise and the MC has a goal to work towards. I'm interested in seeing how the story develops and how the MC gets to find out what to do with the mixtape. Personally I'd suggest prioritizing implementing an MVP version of the music pointing system in the prototype phase to receive feedback early on.