I have a couple suggestions for you.
First of all, culture, the ruler of Prussia should be able to create the Prussian culture, which should be heavy conquest focused, to achieve this change the vanilla Prussian culture to Pruthenian.
And to form the German empire, I suggest giving the king of Prussia a series of decisions to get claims on Germany, first conquer north Germany to form the North German federation. Then decisions to break the territory on the historical kingdoms for both flavor and organization.
Reunite all of Germany to get the German empire, don't make a requirement to have the low countries nor Austria but here's a twist.
If not completely controlled, Austria changes de jure to the Austrian empire, while the Dutch to the Low countries.

That sounds wonderful. I thought I would integrate the culture tradition of Prussia from this mod https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3080163042.
But the historical kingdoms are way too small compared to Prussia, I mean look here https://upload.wikimedia.org/wikipedia/commons/thumb/1/19/Deutsches_Reich_%281871-1918%29-en.png/1024px-Deutsches_Reich_%281871-1918%29-en.png

I probably also have to change the duchys at hthe borders because otherwise the map would look noway like the german reich for example the duchy of bavaria.