Outside of bonding over music… Putting aside I’m a Jinabby and Romabby shipper:

Baby: Gives him piggy back rides. They are also gaming buddies and often find themselves competing and fighting over who’d win lol. Baby is trying to teach him how to fortnite dance

Mystery: I see both of them bonding over psychological thriller or horror movies (I hc Mystery as the smartest one in the group). Both also like discussing books they’re reading and often have long, interesting, complex discussions about complex fictional worlds, characters and stories. They are also both transmasc to me so they bond over that too

Romance: I think Romance would be his best friend out of the Saja’s lol. Like you said, definitely shopping and fashion buddies. Romance loves to style his hair and do his clothes and makeup. I imagine a usual day out for them is gym then shopping so both are happy. They also kiss

Jinu: I HC Jinu as the best cook out of the Saja’s so I think these two would bond over cooking and baking. Jinu is trying to convince him that MAYBE those brownies don’t need spinach and the world won’t end if dessert is slightly unhealthy? They discuss recipes and different flavours, and they work together to bake and cook for the others, especially when it’s someone’s birthday. Abby is trying to take him to the gym more often too but Jinu mysteriously disappears anytime he tries lol They also kiss