Responses
I think about this too, but in my head, Geumryang is holding an old and fragile Taesun in his arms while watching the sunset together. As Geumryang looks at his beloved, he realizes Taesun has closed his eyes forever. For the first and last time, the mountain god cries as he holds his lover tightly in his arms.

Why do I feel like the side-story is going to end with a panel showing us Taesun as old as his grandpa laying on a bed they built on the mountain while Geumrya and the Twins all look young and healthy, crying for their dying father, then suddenly, in the afterlife, the next panel would be Taesun being a child again, reunited with his grandpa and parents?
Then, in the very far future, we see Geumrya in his most natural form, joining Taesun while the twins look up with a family of their own (or simply themselves, they don't need to have a family by biological or emotional means if they do not want to, what I mean is their heart, get it?) Then the last panel would be Taesun and Geumrya smiling, together.