In case it helps — think of stories similar to Backlight
Not bl (shonen ai) but: 19 days
I understand, I really like that trope too, i tried to keep it to ones where the inciting incident was before the start of the story and we learn about it
Highschool/childhood friends seperated
https://www.mangago.zone/read-manga/sono_suki_honto/
https://www.mangago.zone/read-manga/mukidashi_no_koi/
https://www.mangago.zone/read-manga/kujira_no_oshitate_san/
https://www.mangago.zone/read-manga/tabun_korekara_ai_no_hibi/
https://www.mangago.zone/read-manga/oko_sama_box/
https://www.mangago.zone/read-manga/kimi_to_mita_houkiboshi_o_sagashite/
https://www.mangago.zone/read-manga/omega_complex_1/
https://www.mangago.zone/read-manga/ai_mo_kawarazu/
https://www.mangago.zone/read-manga/hello_again/
https://www.mangago.zone/read-manga/koi_o_hitokuchi/
https://www.mangago.zone/read-manga/minori_no_te/
(starting with chapter 2.3)
Towing the line of the trope/ more smutty territory
https://www.mangago.zone/read-manga/bad_friend/
https://www.mangago.zone/read-manga/yasashii_pantsu_no_nugasekata/
https://www.mangago.zone/read-manga/hello_again/
https://www.mangago.zone/read-manga/dogusare_dandies/
https://www.mangago.zone/read-manga/driver_s_high/
https://www.mangago.zone/read-manga/naked_sword_and_tree_peony_of_black/
https://www.mangago.zone/read-manga/admiration_and_longing/
BL where they used to be friends or close with each other, but then something happened that caused them to grow apart/not like each other, and the story is about them obviously growing their friendship/relationship back?