Hyejin is not "high maintenance" or "annoying" for being cautious about her feelings. Dating Haejun back in middle school put her through such an emotional turmoil and led to her apprehension and wariness about dating and love in general. So much that she hasn't dated anyone in those 5 years. If she were to just come and be like "yeah I like you again, let's date" to Haejun when she obviously still feels guilty about their past wouldn't be fair to either of them.
I think Haejun gets that, and he is willing to wait for her to come to terms with her feelings without pressuring her, which is lovely and how it should be tbh.
If you just think that Hyejin is a "moron" for not wanting to rush into a relationship when she's not comfortable about it then clearly you haven't paid enough attention to the story..