For the uninitiated, some neat facts about the Volumes:
1. The reason the Volumes are coming out after so long since the serialisation ended is because Yayoi Sou needed to re-adapt the story from vertical scrolling Webtoon format to horizontal Manga format. That means segmenting, rearranging and lots and lots of redrawing. Heck, she started doing this during the weekly serialisation itself, and was still doing it recently while redrawing for Blue Hearts Vol 1!
2. All 15 Volumes have each contained their own Bonus Report. Volume 15's just so happened to serve as the Epilogue is all (which was ABSOLUTE GOLD. Thank you Yayoi Sou)
For the ones available on Mangadex:
Volume 1 - 19.5: https://mangadex.org/chapter/251598/1
Volume 3 - 52.5: https://mangadex.org/chapter/251599/1
Volume 4 - 68.5: https://mangadex.org/chapter/251600/1
Volume 5 - 83.5: https://mangadex.org/chapter/251601/1
Volume 6 - 97.5: https://mangadex.org/chapter/251604/1
For the remaining ones, fortunately they are very easy to find, albeit not as complete scans.
I would still recommend searching for them though, since they provide nice insight into the cast, not just our leads. In fact, the ones for Vol 12 to 14 makes the Epilogue more rewarding