https://www.mangago.me/read-manga/let_s_make_it_seductive/
https://www.mangago.me/read-manga/boku_no_papa_to_papa_no_hanashi/
https://www.mangago.me/read-manga/i_need_more_than_just_romance/
https://www.mangago.me/read-manga/wait_in_bed_tonight/
https://www.mangago.me/read-manga/ore_no_koibito_no_hanbun_wa_junjou_de_dekite_iru/
Found this person's list https://www.mangago.me/home/mangalist/1740172/
I can only vouch for Mr. Dior and Hige to Suzu to Shabondama. I haven't read any other established relationship BLs.
https://www.mangago.me/read-manga/hige_to_suzu_to_shabondama/
https://www.mangago.me/read-manga/call_me_dior_zhang/
Here you go dear, enjoy ~
Established - married life
https://www.mangago.me/home/mangalist/1608014/
Established - couples
https://www.mangago.me/home/mangalist/2516958/
Do you guys have anything that has established relationships? Like they're alrdy in a relationship