Dude napoleon failed because he expanded way to quickly. any empire that has many different cultures in it make it much harder to control and more likely to just break away again. soft power over proxy governments in Europe is a better way to go for the stability of Germany.

Dude this is historical fiction adding fantasy elements like magic would just make it more generic and less interesting. the entire focus of the novel is the adding of technology far beyond the time periods (early 1400's) scope.

Since your focused on making canals I would suggest Berengar making the rhine-main-danube canal which is located in Bavaria and was created in 1992. with the canal way he could open trade and project power into the Black Sea. the Black Sea is a highly strategic point in eastern Europe and if he projects power into it now before Russia can become an empire he can hold soft and hard power over the region. the amount of trade that passes through this canal between the Rhine and the Danube surpasses the trade volume of even the Suez Canal.

Its probably mortality

Katanas are not fast swords in fact they are heavier then their western counterparts.