hmmm........this is slightly difficult since I do think the better thrillers with slight romance are actually not done by Korea. I actually highly recommend you to check out Chinese, Taiwanese or even Japanese dramas. They really do this amazingly well with constructing a great mystery and having a romantic subplot.
Eiter way for korean;
Confession
Train
Black Out
Walking on Thin Line - this has aired recently and so far for me it's the best thriller drama released in 2025.
Lies Within
these 2 are not underrated but they were not on your watched list so I add those too;
Flower of Evil
My Name
And a very controversial drama, that goes strongly into the arthouse direction but that is really good in terms of uniqueness, thriller and romance vibes.
Somebody
