k/jdramas where the leads are childhood friends

I recommend checking out Weightlifting Fairy Kim Bok Joo!

The main leads met in elementary school and became friends, but they got separated until they met again in college. That's when their love story begins. :)

maybe Baker King Kim Tak Gu (she loves ML at first but ends up with 2nd ML in the end they all meet as kids), Jang Ok Jung live in love, and maybe film My Little Bride

• Healer - very young childhood 
• Hello Monster - FL was the ML's "stalker" / sort-of-friend
• I Hear Your Voice - the leads met in childhood; the FL helped the ML and the ML vowed to protect her
• I Need Romance 3 - the FL looked after the ML when they were kids
• Pinocchio - the leads grow up together

Also this one is from Taiwan and the leads meet in high school, but I strongly recommend In Time With You. The ML develops a crush on the FL in high school, where they are sort-of rivals (mainly from her perspective). He sort of starts a friendship with her, but for fear of losing her he starts it by saying that he'll never have feelings for her (in the moment she's a bit biased against love). Later in life they become best friends. It requires a little bit of patience, but since you specifically wrote that you're open to suggestions where the FL dates someone else first, I thought I'd mention it. 

Bubblegum, She was pretty, In time with you (tw drama), Pinocchio