I'm wondering at what point, if ever, the Czechs of the Austro-Hungarian empire could have been Germanized.
Frederick the Great is killed in the Seven Years War. Austria recovers Silesia. When the Bavarian line of the Wittelsbachs dies out in 1779, Archduke and Emperor Joseph II executes the deal he tried OTL: ceding the Austrian Netherlands to the Zweibrucken Wittelsbach heir in return for Bavaria. (OTL it was blocked by Frederick, and Joseph's mother Maria Theresa was opposed to any fighting for Bavaria.)
The French Revolution, the rise of Napoleon, and the mediatization of the HRE follow mostly as OTL; but with Austria already holding Bavaria, the scattered Hapsburg lands on the upper Danube ("Hither Austria") are not written off as OTL, but retained and the surrounding petty states are absorbed into Austria.
France defeats Russia and Austria, and revives the Kingdom of Poland, including Galicia. When the wars finally settle out, Poland survives as an independent state under Napoleon's brother-in-law Murat. Napoleon himself reigns in France. The Bourbons rule Spain and Naples. Saxony expanded as an ally of Napoleon, then switched sides and got to keep its gains. (Napoleon was beaten out of Germany in 1814, but had previously beaten Russia so badly she stayed out of the later war.) Napoleon awarded the Kingdom of Italy to his younger son. The Hohenzollerns had an accident; Saxony grabbed Brandenburg, while Franconia went to Austria.
(Just to fill in the rest of the settlement: the Wittelsbach state includes Belgium, Luxembourg, the Palatinate, the west bank of the Rhine, Berg, Mark, amd Mayence. The Kingdom of Hanover, under George III's second son (Prinny died shortly after fathering Charlotte; George III died in 1804), includes Oldenburg, Hanover, Brunswick, Westphalia, and Holstein. Saxony includes all Wettin lands, Brandenburg, Pomerania, and Mecklenburg. Hesse-Kassel, Hesse-Darmstadt, Nassau, Baden, and Wurttemberg survive as "petty states".)
Austria has a crisis in the 1840s, which leads to Hungary breaking away.
What does all this mean for the Czechs? That the Austrian state surrounds the Czech lands on three sides, and is overwhelmingly German. The only non-Germans are the Czechs and Slovenes. In the late 1800s, rural Czech-speakers migrate to the new industrial cities of Austria. Not just those in Bohemia, Moravia, and Austria but also into Silesia, Bavaria, Swabia, and Franconia. Thus dispersed, they quickly forget their homeland speech and adopt German. Others emigrate overseas. By 1900, Czech speakers are a minority even in Bohemia. In spite of an attempted "Czech revival" in 1880-1920, use of the language steadily declines. Intermarriage also has its effect.
By 1950, only about 10% of the people speak Czech as a primary language, and only about 25% speak it at all. By 2000, 2% and 10%.