bl recommendations of the mcs being in a musical band... not idol group.
https://www.mangago.me/read-manga/dark_heaven/
one of favorite bls!!
https://www.mangago.me/read-manga/hishakaishindo/
only one is in a band but a great story nonetheless
https://www.mangago.me/tag/band/?page=1
maybe you could find some more here
SIDE STORIES.... VOLUME 2 PLEASE PLEASE PLEASE PLEASE PLEASE I CAN'T LET IT GO I'M GONNA KMS