Just a thought I had last night, not sure if it was intentional or it just went over my head, but I think the Dark Sun is supposed to symbolise the shrine maiden's glance and the character's longing to be understood/not alone at the end. Part of the folklore of the mountain is that, during sunset, the mountain's true form can be seen, as if illuminated by the dying sunlight. In other words, during the witching hour, our world is at its closest to the Other Side. Witnessing it, Mt. Hikami's true form, reminded me of seeing into a person's soul and all of their secrets, which was soothing to those who'd chosen to die. I think it was also the time outsiders were accepted onto the mountain, which makes me think the later hours are when our souls are closest to death, allowing them to be read easier.
A similar taboo was also associated with the shrine maiden's glance as viewing the sunset; those who didn't wish to die should avoid eye contact, as their glance was believed to steal their soul. It's part of my headcanon that the stronger maidens wore veils as their gaze literally developed a bewitching effect like the sunset, partly fuelled by their attack where they remove them, exposing you to blinding light, like looking directly at the sun.
Ultimately, I think that's why the characters are so drawn to it. Similar to Blooming, being glanced is both soothing and terrifying, bringing you closer to death (Miku dissolving into the light) or to the living through understanding (Yuuri/Hisoka forever). What do you guys think?