I second How to Get My Husband on My Side, more the novel than the manhwa but both are great. Your Throne is the classic, but there's also Kingdom of the Queen which is super underrated. If you want another one that is a little less plot heavy but what makes it good is the fact the MC isn't a freaking idiot who survives based on solely MC buff, then I'd recommend Talented Baby Squirrel. My absolute favorite will always be Kill the Villainess. Note, that once you read it there's really no going back tho, like you'll forever be wandering looking for something that good again and it literally doesn't exist. If you're okay with BL, Turning is pretty plot heavy too (at least the novel is). (All links are below)
A few others I personally think you'd like (mix of genres and mcs):
1. Northern Blade - Plot heavy, Male MC, trying to revive his sect after he was betrayed. Murim but not super complicated BS power structure, focused more on the politics than the martial arts. Art is top tier and plot is *chef's kiss*. Little to no romance, it's there but it doesn't infringe on the story or is a main part.
2. Privileges of the Reincarnator (Preference for the Possessor) - This one is a bit weird. Female MC, but it gets DARK at one point even though it starts relatively light hearted. I read the novel after S1 ended and I can say that by the mid-end of it there's a plot twist that personally enraged me, and it kind of turns a lot of stuff on it's head. Haven't caught up with the manhwa, but if it follows the novel you're in for a treat.
3. Secret Lady - Holy FUCK the art. The plot is also crazy good, and a bit convoluted, but note that the art style is similar to Stepmother's Marchen. Saddly on hiatus due to artist health issues, but still worth it.
4. Park Hanhoo's Manager - BL. However, plot is also just UGH. Starts off with a serial killer, and the whole story is him trying to hunt him down. Underrated.
5. Survive Romance - Female MC. Surprisingly zero romance. Zombie apocalypse meets shoujo meets time loop. HIGHLY recommend. MC is freaking UNHINGED.
6. This Witch of Mine - Multiple MC. Bring tissues. Romance heavy, and facing the inevitability of immortality and loving a mortal person. All characters in this series are amazing and deserve more than what they got, but not all stories get a happy ending. Or at least, a happy ending is very different depending on the person.
7. As the Heart Leads - Kind of a combination of Secret Lady, How to Get My Husband on My Side, and Please Marry Me Again Husband. Sacrificial FMC. ML who is actually an 8000000/10 and does NOT deserve the shit happening to him.
That's it! I've got more, but hope you enjoy them.
Links:
Your Throne: https://www.mangago.me/read-manga/i_want_to_be_you_just_for_a_day/
Kingdom of the Queen: https://www.mangago.me/read-manga/kingdom_of_the_queen/
Talented Baby Squirrel: https://www.mangago.me/read-manga/talented_baby_squirrel/
Kill the Villainess: https://www.mangago.me/read-manga/kill_the_villainess/
Turning: https://www.mangago.me/read-manga/turning/
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Northern Blade: Northern Blade ( https://www.mangago.me/read-manga/legend_of_the_northern_blade/)
Preference for the Possessor: https://www.mangago.me/read-manga/preference_for_the_possessor/
Secret Lady: https://www.mangago.me/read-manga/secret_lady/
Park Hanhoo's Manager: https://www.mangago.me/read-manga/park_hanhoo_s_manager/
Survive Romance: https://www.mangago.me/read-manga/survive_romance/
This Witch of Mine: https://www.mangago.me/read-manga/this_witch_of_mine/
As the Heart Leads: https://www.mangago.me/read-manga/as_the_heart_leads/

'Serious' Plot heavy reccs needed like 'steomothers marchen' and 'the flower dances and the wind sings '