Some are still marked as ongoing but just because the status is not updated yet...
https://www.mangago.zone/read-manga/the_fake_princess_op_bunny/
https://www.mangago.zone/read-manga/please_throw_me_away/
https://www.mangago.zone/read-manga/villainess_maker_1/
https://www.mangago.zone/read-manga/happy_ending_for_the_time_limited_villainess/
https://www.mangago.zone/read-manga/the_evil_lady_will_change/
https://www.mangago.zone/read-manga/the_admiral_s_monster_wife/
https://www.mangago.zone/read-manga/lady_beast/
https://www.mangago.zone/read-manga/adeline_s_twilight/ mainstory is already completed, just some more extras might still be added soon https://www.mangago.zone/read-manga/the_stereotypical_life_of_a_reincarnated_lady/ art is unusual but charas & story makes it worth the read
https://www.mangago.zone/read-manga/this_is_an_obvious_fraudulent_marriage/
https://www.mangago.zone/read-manga/the_precious_sister_of_the_villainous_grand_duke/
https://www.mangago.zone/read-manga/the_younger_male_lead_fell_for_me_before_the_destruction/
https://www.mangago.zone/read-manga/the_evil_lady_s_hero/
https://www.mangago.zone/read-manga/the_villain_demands_i_love_him/
https://www.mangago.zone/read-manga/i_got_married_to_a_villain/
https://www.mangago.zone/read-manga/divorce_me_husband/
https://www.mangago.zone/read-manga/the_tyrant_husband_has_changed/
https://www.mangago.zone/read-manga/my_husband_hides_his_beauty/ can't remember them ever mentioning the ml's title but power & independency are duke-like at least
https://www.mangago.zone/read-manga/sincerely_i_became_a_duke_s_maid/ ml is cursed
If reincarnation / transmigration without further fantasy elements also fit your bill, then
https://www.mangago.zone/read-manga/la_dolce_vita_di_adelaide/
https://www.mangago.zone/read-manga/leveling_my_husband_to_the_max/
https://www.mangago.zone/read-manga/the_reason_why_raeliana_ended_up_at_the_duke_s_mansion/
https://www.mangago.zone/read-manga/daisy_lisabel/ ml is duke's heir
https://www.mangago.zone/read-manga/is_it_a_fortune_or_is_it_a_woe/ ml is duke's heir
https://www.mangago.zone/read-manga/the_duchess_50_tea_recipes/
I’m going to save this. Thanks!
You're welcome. Have fun reading ヾ(❀╹◡╹)ﾉ~
Romance Fantasy recommendations of FL x Duke that are complete?