Uminekos aren't exactly known for their player interactions, they are more like crime novels but where the player is intended to try to solve the mysteries by themself. So I don't know what kind of gameplay would be appropriate for such game without feeling out of place.
Renpy support looped BGM with intro, it is as easy as:
play music "intro.ogg" noloop
queue music "loop.ogg"
By the way, a lot of royalty-free musician don't provide distinct intro and loop files, they instead use tags like LOOPSTART and LOOPLENGTH so the bgm will loop in engines that support those like RPG maker. Using Audacity (and a sampling of 44100), you can find back what the original intended loop was and easily create intro and loop files for renpy.