Him not using the slime might be contributed to the fact that summons lowers his max mp, and having two right off the bat just means fewer spells to cast. Or I'm just using too much brain cells on an isekai and the author just actually forgot about it.
His sinister plan to become the demon king's daughter can't come true! It'd be terrible if a romantic interest had those big fish lips. Also that was quite the O-face.