Is it more militaristic bcoz it have more officers and soldiers or does it have more officers and soldiers for being more militaristic?
Could the Prussian spirit be simply translated by spawning more soldiers and officers?
That's a good point. Everything isn't material, we can agree on that. Those immaterial things like pride and confidence are much more difficult to render in a game than the quantifiable things.
However, how did this militarism affect Prussia other than by making it very difficult to pacify (in your view)? Even if it's only that, then you would be forced as a player to engage more with the army than someone playing as, say, Portugal. Maybe Prussia could also start with some "good" military laws (if you insist on saying they were better than comparable nations).
We know the interest groups currently can draw support from many types of pops. A modifier making it more likely for Prussians (alone) to support the military would be horrid. We have to have a reason for this, something replicable over the course of either a few years or a generation. Maybe "recent invasion of the country" and "recent victories" could work. After all, the Prussia of old was obliterated by Napoleon (slight exaggeration here, sorry) and was left as a rump State, which managed to reform itself militarily and to win a decizive victory against France.
Something akin to the "revanchism" mechanic from EUIV, cristallized by a stunning military victory and a very favorable peace treaty (Prussia gained Westphalia), could probably be seen as an in-game cause of Prussia's military popularity.
However, we then turn to Austria and Russia, who both gained a lot from the Napoleonic wars. For Russia, it was still an autocracy and the Tsar saw himself as the gendarm of Europe until the Crimean War, so the narrative fits. Nobody said that a militarist faction in power necessarily means that the good laws and social organizations are there to back it. For Austria, it was treated far more gently by Napoleon (lost less territories), so maybe it wouldn't start with such a modifier.
In addition, we could also explain France's relative weakness during the era by reffering to the wars of the Revolution and the Empire, saying that France was exhausted demographically and its intelligentsia was disheartened by its ultimate defeat, which wasn't followed by some eclatant victory.
Here is a map of Europe in 1812 to show how butchered Prussia was and how comparatively intact Austria was. (That said Prussia’s conquests that were removed by Napoleon were recents and were replaced by Westphalia)