preferably no mangas

Nana

https://www.mangago.zone/read-manga/living_will/ short but tragic (not toxic tho)

Non-manga:
https://www.mangago.zone/read-manga/a_story_concerning_sweets/ 
https://www.mangago.zone/read-manga/hone_no_nazuki_made_aisu/ (toxic)
https://www.mangago.zone/read-manga/final_film/ (sort of toxic)

Manga:
https://www.mangago.zone/read-manga/shiroi_heya_no_futari/ (the OG doomed yuri)
https://www.mangago.zone/read-manga/oniisama_e/ (another OG doomed yuri)
https://www.mangago.zone/read-manga/kimi_to_tsuzuru_utakata/
https://www.mangago.zone/read-manga/this_love_that_won_t_reach/ (rushed and ambiguous end)
https://www.mangago.zone/read-manga/kimi_ga_shinu_made_koi_wo_shitai/ (on-going; doomed side couples and possible doomed main couple)
https://www.mangago.zone/read-manga/zenbu_kowashite_jigoku_de_aishite/ (on-going but could end up being doomed; toxic relationship, child abuse by parent)
https://www.mangago.zone/read-manga/efude_to_kurokinu/
https://www.mangago.zone/read-manga/141108/
https://www.mangago.zone/read-manga/aliens/
https://www.mangago.zone/read-manga/the_story_of_the_princess_and_the_little_mermaid/
https://www.mangago.zone/read-manga/a_bud_in_secret/
https://www.mangago.zone/read-manga/she_says_goodbye/