I don’t think they were separated for that long. The time skip were 8 years because it c...

I don’t think they were separated for that long. The time skip were 8 years because it counted from when they started college.
Hyung was there after Sunny finished his military enrollment. He was also there when Sunny graduated and started working. Sunny event shared the news of his overseas post with him.
Sunny must have been 26-27 when he graduated (Korean age starts from 1 and most people go to college when they are 20. Most schools do 4 year course. Plus 2 years in the military.) He is now 28.
My theory is that hyung started the plan to separate after he heard of Sunny being sent overseas. That’s probably also why Sunny decided to propose before he left.
If my theory is true, they were separated for 1-2 years at most.