I just randomly watched C-Drama title Hidden Love, the main couple also have similar plot. Female lead is 14yo and have crush on male lead, but male lead always sees her as a kid, as his bestfriend's sister, or almost as sister. But years passed and female lead become young adult, and after some moment together male lead didn't see her as a kid nor sister anymore, he begin attracted to her.
If Go Ahead take more time, and natural way to develop main couple's romantical feeling like in Hidden Love, i probably give Go Ahead's rating higher.
Those 20 episodes of adult timeline can be well spent if focus more on Ling Xiao and JianJian rather than expanded to the story of JianJian' best friend. Especially Tang Can's romance story, it's unnecessary.
I know those 3 bestfriends involved in love triangle to shake their friendship, but i just think it's unnecessary because there are still other issues.