Responses
I feel like you are on to something. The King in the vision isn't the MC because we can see from Chapter 1 his form is human sized. The one in the Vision is more of a giant so unless he has an evolution or something it shouldn't be him.
I feel like maybe the one to kill the demon king will be himself, the current timeline demon king will kill the future timeline demon king that is resting in the tower.
One because our MC is selfish like that and wants to be the only demon king.