I think the author just wants her to be a bigger villain than that
Like usually the story would go "the little grand duchess yields after losing and the grand duke was actually the real villain the whole time" but I think since we know he's already willing to give up his soul stone just for the poison wizards help, the duchess is supposed to be the bigger villain at least for now. Author prob just didn't want to go that young women characters will make friends despite starting on opposite sides path lolol
But I def agree she's a bit boring. Though I am kinda interested to see how messy she'll get to get what she wants since it seems like the dad wanted to keep her from getting too messy given him keeping the thing from before a secret
I was hoping for the character of the grand duchess to be deeper than that and that she would recognise the empress as she won but clearly she’s a lost cause and a boring character who only thinks of killing