I would say after reading almost half of the novel, i'd say that the grand duke DOES have more power than the kings cus they have more at stake in the whole situation, the kingdom and their race's survival and happiness is important for them. The "game" gives them the power to basically over throw any kings that is not to their liking. For a lot of the kingdoms, these kings aren't even their firsts. Cus they are stuck in that world in a limbo state, they start over and over again to meet the perfect king. So if the king even makes the wrong decision by mistake, it'll be hard to gain back the trust cus those grand dukes have seen these sort of shit before and are basically over it. They don't have the patience to babysit anyone incompetent designated to rule over them and would rather start over with someone new.
However, when they meet THE king, they would never let go of you. For example glasses dude and the MC. Their dukes are obsessed cus after going through so many incompeted kings, they are the closest to their ideals. Who would ever let that go and go through that cycle of sifting through new kings again? Honestky MC and glasses dude's situation are kind of the same cus they both quit the game due to the grand duke and it was seen as betrayal and abandonment by their vassals and grand duke. So when they were sucked into the world, the grand dukes aren't willing to let them go.
bro the grand duke's here are so obsessed with their 'kings'