Sounds exactly like one room TA, have you read it already?
bl:
https://www.mangago.me/read-manga/stigma_marloong/ (noir, very gritty)
https://www.mangago.me/read-manga/yang_il_woo_and_i/ (high school, mc hasn't gotten to the leaving part yet)
gl:
https://www.mangago.me/read-manga/run_away_with_me_girl/ (woman ditches misogynistic fiancé for childhood best friend)
het:
https://www.mangago.me/read-manga/tears_on_a_withered_flower/ (married woman x yonger guy)
perhaps you've read this already, and the MC's not the one in the toxic relationship, but here
https://www.mangago.me/read-manga/sabita_yoru_demo_koi_wa_sasayaku/
Problematic leadership activities!
recommendations where mc is in a bad/toxic relationship with their partner but then they meet another person with whom they fall in love and eventually leave the bad/toxic relationship? can be bl, gl or straight