I can read these mangas many times! I rate them >9.5/10 based on my own preferences, so there will be lots of non-mainstream BL
Genres: NO PWP, no doujinshi
Length: <100 chapters or 3 vols
Search by tags: http://www.mangago.me/home/people/12706/manga/1/
If you like this list, please click "RECOMMEND". Thank you XD
2nd list: http://www.mangago.me/home/mangalist/655038/
3rd list: https://www.mangago.me/home/mangalist/3746699/
(IF 1ST LIST IS FULL, I'LL MOVE OLDER ONES TO MY LIST 2&3)
Basically all of them have a poly ending, apart from a few where it's not clear if one of them is gonna be endgame or all of them
Beware of SPOILERS
OTHER LISTS:
Reverse harem in general (BL): https://www.mangago.me/home/mangalist/2486828/
Polygamy (FL): https://www.mangago.me/home/mangalist/2375222/
Reverse harem in general (FL): https://www.mangago.me/home/mangalist/2374714/
Throuples: https://www.mangago.me/home/mangalist/2688301/?filter=&page=1