Thank you for the suggestions, everyone! I have at long last finished the mechanics-based section and stripped out every trace of the base game setting. The hardest part was probably the bit where I spent 4 or 5 days reading about 17th-century HEMA in an effort to translate the various national styles of fencing to the fencing school gameplay mechanics. I've taken a few liberties--namely, French fencing has certain aspects and authors overemphasized, because from what I understand, French fencing was very similar to Italian fencing until the latter half of the century. Additionally, for Arabic and Turkish fencing, I've adapted
furusiyya as a sort of jack-of-all-trades fighting style, de-emphasizing horseback fighting some to fit the game better. The Nordics and Russians don't have styles, which is more or less fitting with the base game setting (where Not Russia has no fencing school and Not Scandinavia is simultaneously the Netherlands and also late-era Vikings.)
My favorite fencing mechanic that I came up with was giving the Dutch fencing style, based on
Gérard Thibault d'Anvers' Academie de l'Espée, an ability where you're more likely to hit your opponent the better your character is at math.
I also wrote up a gigantic language chart listing not only the various languages one's characters may speak but how many skill points you need to learn other languages based on your first language. (Russian got the short end of the stick :v) Had to include "miscellaneous" columns clarifying that other languages exist but I didn't have the space to put them on the chart--I had room for the big ones (French, English, German, etc.) but I had to squeeze a few things together for simplicity's sake (mutually intelligible or partially mutually intelligible languages, e.g. Portuguese/Spanish/Catalan, Swedish/Danish/Norwegian, Russian/Belorussian/Ukrainian, are combined into single languages, for example, and I've got a list of other possible languages that had to go unlisted entirely.)
Now I only have to write a few dozen pages about European history and 17th-century society, ha. The book does a kind of paint-by-numbers thing where they spend 4 pages on the game world's history, then devotes 1-3 pages each to the seven nations, but since there are significantly more than 7 nations in Europe, a lengthy, non-imaginary history, and since I'm a history major, I'm thinking I'll go a bit more in-depth for any players who aren't familiar with the period. I think I'll put down a more or less chronological account of European history, starting in England and France at the close of the Hundred Years' War, then jumping about to discuss topics like Columbus' voyage, the Protestant Reformation, the dissolution of the Kalmar Union, the formation of Poland-Lithuania, the rise of the Ottoman Empire, the Tudor succession struggles, the Schmalkaldic War and the Peace of Augsburg, and so on until the "current" topics, like the 30 Years' War, the Polish-Swedish War, the conflicts of Charles I with Parliament, and the like. The latter of these sections will probably be organized by region or country (e.g. British Isles section, Central Europe section, Baltic Sea section, etc.) and describe not only the larger political picture but society and everyday life in these places.
Thinking of a more adventurous sort of plot than the one I'd originally envisioned, probably one set outside Germany but where the players definitely feel the effects of what's happening there.