The elephant in the room here is "balance of power". It was specifically designed to prevent this sort of scenario, as any country that gets too uppity will find itself opposed by a multi-state coalition that will put a stop to any European conquest shenanigans.
Maybe if Metternich falls down some stairs and dies in 1813, then Hardenberg will have much more success at the Congress of Vienna. Prussia gets Saxony and the preponderant role in the German Confederation's Diet, and France gets dismembered. This sets Prussia up nicely going forward, but they still need to industrialize. But really, once the terms of Vienna are in place, the conquest of all Europe becomes unmanageable.
Maybe if Metternich falls down some stairs and dies in 1813, then Hardenberg will have much more success at the Congress of Vienna. Prussia gets Saxony and the preponderant role in the German Confederation's Diet, and France gets dismembered. This sets Prussia up nicely going forward, but they still need to industrialize. But really, once the terms of Vienna are in place, the conquest of all Europe becomes unmanageable.