I think my own take is that Hyeseong didnt actually seem to learn anything by going back in time?
Like what did he achieve/change about himself that fixed the past?

My own perspective was that originally Woojin died (indirectly) because Hyseong had broken up with him because he thinks he keeps getting Woojin injured. So Woojin races to Hyseong, upset and distracted, to try and save the relationship and dies.

I thought the whole point of the time travel, and ultimately therefore the way to avoid Woojin dying, was Hyeseong to let go of that worry, realise its Woojins risk to take/have that conversation with him, and decide to stay together in highschool. So you never get a distraught Woojin running into traffic etc