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/
(If 1st list is full, i'll move older ones to my 2nd list)
Long series: http://www.mangago.me/home/mangalist/645867/
the desc may not be accurate
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◍1- 66 : older seme x younger uke
◍67 -187 : older uke x younger seme
◍188 -199 : shounen ai
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the second list:
[~༶◍✧*。Age Gap (two) ｡*✧◍༶~]
https://www.mangago.me/home/mangalist/1800386/