Many people associate MiziSua with the Sun x Moon dynamic as for Mizi's cheerful and outgoing personality in contrast to Sua's evasive and silent personality but it also works in a way that Mizi was Sua's sun
The main reason for Sua to sacrifice herself and Mizi just accepting it was because Sua couldn't live without Mizi and would probably be worse than Mizi if she was the one who came out alive from the first round. Sua was dependent on Mizi for her mental well-being, despite hating the fact that she was like her puppet she loved Mizi more than herself
Mizi knew this and let Sua sacrifice herself, not because she "wasn't as desperate as her" as she confessed to Hyuna. She WAS desperate. She was as desperate as her. Mizi couldn't stand seeing Sua's face drowning in tears as if it was the end of the world so she chose to protect the happy face of Sua, even if that happy face belonged to a dead body. She would rather for Sua to die thinking that she saved Mizi than be swallowed up with guilt and mentally in shambles.
Mizi is Sua's sun because Sua simply couldn't do it without her, she wouldn't have her light to illuminate. Without the Sun, the moon is quite literally a rock in the sky but the Sun would still be relevant without the moon.
But human beings aren't celestial bodies. The sun and the moon doesn't have an intense emotional bond between each other but Mizi and Sua do. This time, when the moon ceased to exist, the sun started to destroy itself to eventually create a big explosion
