> I'd pay a premium right now if it could help me review Kanji and vocabulary in a non-boring way! (eg. calculate a difficulty score for each manga, determine which manga have more vocabulary and kanji you know, etc.)

FYI, I don't have manga yet, but a website that I'm running (https://jpdb.io) has exactly that for other types of media (anime, movies, light novels, visual novels, video games, etc.)