Sorry I haven't seen enough cdramas to recommend those, but here are some kdramas:
Romance is a Bonus Book, the second leads are well-rounded, interesting characters who work through their feelings about the first leads and seem to be attracted to each other at the end.
Sweet Stranger and Me, the second leads are selfish, obnoxious types. I find them hilarious. He's rich and spoiled and wears very expensive clothes, and she's financially strapped and is forced to make economical fashion choices. At the end it does seem like they are headed for a relationship.
Itaewon Class, the first female lead and the second female lead are both complex and morally grey. The ML has loyally loved one of them for years, but they've never started a relationship. The other woman comes along and attaches herself to him. The romance isn't the dominant thread in this drama. It's a revenge story. But the way the drama makes it difficult to be sure who the first female lead is - I think you would go for that.
My Lovely Sam Soon, the second female lead is a truly lovely person, who for reasons it's not necessary to spoil, suddenly left Korea for the United States. She arrives back in Korea with an Asian-American friend who is into her, but she just wants to resume her relationship with the first male lead. The second female lead and the first male lead actually do go on some dates, but it turns out that he can't give the first female lead up. So she goes back to the U.S. with her friend, and it's strongly suggested that his loyal attentions and support will finally be rewarded. This drama is amazingly well written and funny, and also touching, with great characters.
Our Beloved Summer, the second male lead is sadly smitten with the first female lead for most of the drama, but all along the second female lead, his junior colleague, is seen supporting and helping and liking him. And at the end, she does confess to him, and it's implied that they will get together.
The Time We Were Not in Love (kdrama) and I Don't Love You Yet (jdrama) both have an interesting plot where the leads are lifelong friends who have never taken the step to become lovers. In these dramas, the FL has a relationship with someone else who is totes not as good for her as the male lead would be. Happily, the wrong-for-her relationship ends and she gets together with the male lead.
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It's not exactly what you're asking for, but there is some ambivalence about who the first and second female leads are in Oh My Ghost, a ghost love story with some cute posession scenes and a lot of cooking. Since one of the female roles is a ghost, and, well, you know ghost stories - the only ending possible is some variation of the ghost finding peace and passing onwards. So it's not possible for her to end up in another happy relationship. However, at the end the leads do put their own relationship on hold to dedicate themselves to in a way finishing the ghost's life goals by saving her family's restaurant and taking over the mentoring of her surviving brother. One thing I wonder about after reading your request is if you might like stories that allow the leads to not be so 100% focused on each other as the one, perfect, true partner. So you might like the fact that in Oh My Ghost, the male lead sincerely loves both women - and it's not adultery because the women keep the secret from him that one of them is allowing the other to possess her body. I know it sounds disturbing and adult, but it's all pretty innocent. Even though the leads aren't actually together at the end, their relationship has so much going for it that I felt sure they would be together later on. It truly felt to me like they were first going to dedicate themselves to the ghost's family's needs, and were waiting out of respect for her. It's been awhile since I watched this drama, and I know the ending I'm describing is unusual, but it felt like a happy ending to me.
emeraldarrows:
