Mature FL/Meet Again/Friendship & Romance

These fit some but not all of what you're looking for:

• Dear Mayang Street  - same FL as Go Ahead and she's more mature here than in that, imo (the drama also has a similar vibe, to me). Friends group that grows up together, slice of life, leads separate then reunite years later. There is a heavy love triangle but it's not annoying and very poignant, imo.
• A River Runs Through It  - friends group that grows up together, FL is immature at the beginning but matures over the course of the drama, the leads break apart due to the ML sort of giving up after years of a one-sided love, then reunite. There is a love triangle, but it's one sided (FL likes the SML but he always likes the SFL, and the ML always likes the FL until she likes him back).
• My Fated Boy - the FL is mature and the leads grew up together (FL is seven years older than the ML), separated for a while, and then reunite, leading to the romance.
• Lovely Us - no separation/reunion and the leads are young, but it's a friends group who have grown up together.
• Meeting You - FL is young but mature, the leads were friends as young kids, separated, and then meet again in high school. The exact opposite of FL dependent on the ML (he's very dependent on her), no love triangle, very slice of life and healing, imo.
• Blue Birthday/Blue Birthday (I prefer the Japanese version but the kdrama one is good, too), a bit like Shining for One Thing. The jdrama version focuses more on the friends group. There is a mystery aspect to the plot (not sure if the no police/detectives extends to that genre so just mentioning it to be safe).

These may meet your criteria:

• Let's Meet Now ML and FL were childhood friends, separated for many years, meet as adults but she doesn't recognize him. They reconnect and become closer.
• You Are My Glory ML and FL were high school classmates and FL confessed to ML (and was rejected). Now working adults, FL and ML meet again, get to know each other and fall in love.
• Legally Romance ML and FL were college classmates and rivals in the classroom. Ten years later, FL works for the ML in a law firm. In an accident, she falls into a coma and she relives their college days. But she's learned something this time around so she rejects her college boyfriend (SML) and enjoys her college days with friends. She also finally figures out that the ML has been her main support during this time, but she was too oblivious to realize it then.
• Our Secret ML and FL had met when they were about 5, and their grandparents engaged them to one another. They eventually became neighbors in high school and become desk mates.
• Love Is Sweet FL is hired to the same firm ML works for. They were childhood friends but he cannot show any favoritism towards her in their shared environment.