Artist that works with a writer, gave it a quick review over breakfast. If I was your artist I'd appreciate your openness to interpretation!! Personally I would enjoy playing with the pacing a bit. The amount of description you gave worked well for me, I feel like I could imagine what was going on and would have been able to come up with thumbnails without any trouble.
Anyway.
I found the demon king's dialogue just a tiny bit over wordy. It didn't feel like an interesting character tic or you would have pushed it more I think. There was one line in particular (I cannot fathom who you must...) that felt odd to read out loud to me. I am not a writer so take it with a big grain of salt.
Overall this read like an introduction (as you said), not a short story. I think I would have enjoyed more interaction between Sun and the demon king and less chatting about the escaped soul but if the phantom continued version is focused a lot on the demon king then it makes sense to spend this time establishing him.
If there was a continuation, I don't think it matters whether readers have background knowledge about Sun - mc reveal can be dramatic enough. As whether it helps give you a satisfying conclusion here, doesn't make terribly much difference to me. You've got a guy who's kind of silly, super powerful and wants something that sounds absurd. Cool.
This type of story isn't an S or A tier match for my tastes but if the art was cool I'd keep reading to catch more character interactions to find out if it was a story for me. I don't think you had anything wildly out of the ordinary that hooked my attention but you didn't lose me either. I like a playful/overpowered main character well enough and this seems like a fine set up for an action comic with some humor.
Hope any of that helps!