I disagree. Annexing the Czech lands, or at the very least setting up some kind of autonomous federal unit separate from German Bohemia, within the empire, would be extremely likely. It was part of the Holy Roman Empire, the German confederation, and “Austria proper,” so letting it go would likely anger the Austrians. Not annexing it also makes the eastern border considerably longer than it needs to be, though admittedly less of an issue being surrounded by puppets. You also have to consider that the Czechs may see the old kingdom of Bohemia as theirs, in entirety, and present a risk to the Bohemian Germans in a future conflict. And despite the nationalist rhetoric of the German unification, it didn’t stop them from taking on a bunch of poles, nor did it stop them from annexing a bunch of Frenchmen in the Franco-Prussian war. The Czechs are nowhere near populous enough to be anything more than a sizable ethnic minority, which may even benefit certain parties in the German government considering many may prefer Czech ethnic parties instead of rival German political parties. And despite the non German nature of the territory, if the German empire annexed German Austria, but left the Czechs out of it, I’d argue many Germans would be pissed. As part of the Holy Roman Empire and the German confederation, the Germans would’ve considered those territories German by right, even if not by nationality, and it would be a national embarrassment to willingly refuse the political unification of the old Holy Roman Empire, when given the opportunity, as that was the entire point of German nationalism in the first place. I’d almost argue it’s more likely for the German empire to willingly refuse to annex the Germans in western Hungary, although that too is unlikely. Furthermore, from the Kaiser’s perspective, annexing the entirety of the old HRE would completely cement the Prussian/Berlin-based dominance of ALL of Germany, and make whichever Kaiser who accomplished it the hero that German children hear about when they’re put to sleep. After beating the shit out of France, twice, carving western Russia into pieces, dominating the old ottoman balkans, and finally reuniting the entire German Reich, at the expense of the Austrian’s non German empire, it would be immensely symbolic as the moment Prussia/Germany has beaten every single one of its traditional foes, and established Germany as the unquestionably dominant power of continental Europe.