If I wanted to actually learn about ritsumao's relationship in canon, which stories do you recommend I should read?
OH I AM SO GLAD YOU ASKED !!!!!!!!
Absolutely the number one most important story is Band Ensemble. If you only read one ritsumao story, it has to be this one. I can't say too much without spoiling, but essentially it gives a very in-depth look at both of their feelings about each other, and serves as a major turning point in their relationship in early ! era (to the point where I will refer to ritsumao moments as being pre- or post-band ensemble and the people who are familiar with them know exactly what that implies lol)
Next most important one in my personal opinion would probably have to be Janus, which is their look back story and shows us how they first met and became close as kids. Also gives very important insight into both of them as individuals, especially when it comes to their relationships with their respective families
The rest aren't in any particular order, but I'd also recommend Horror Night Halloween, Chanbara Night, Fleur de Lis, and Sweet Hunt
Horror Night Halloween is primarily focused on Izumi and Makoto's relationship, but it also includes some very important scenes with Ritsu and Mao that show how much they've changed and grown with each other in the months since Band Ensemble
Chanbara Night shows how the two of them act when they're made to directly compete with each other. Trickstar and Knights are rival units of course so its not the first time they've been on opposite sides of a competition by any means, but it is the first time they've gone head to head one-on-one and I find it really fun for that reason
For Fleur de Lis, Mao doesn't actually make an appearance but I still see it as being very important for ritsumao because it's another story that really delves into Ritsu's growth post-band ensemble with him learning to be less codependent and reliant on Mao
Sweet Hunt honestly isn't that important in the grand scheme of things but I still would recommend it because I think it's really cute lol. But other than pure cuteness, it also kind of explores the ways their relationship has changed since the establishment of Ensemble Square, them both becoming third-years, and moving into the dorms which all contribute to them not having as much time to see each other
Oh, also, definitely not very important by any means nor focused on either of them whatsoever, but I would also have to recommend Tanabata Seven Bridges if only because it includes the "maakun, lets get married" "ask me again when its legal in this country" scene lmao
Those are all of the ones that come to mind for me but fellow ritsumaop's please feel free to contribute in the notes :3c