I totally agree; the durability of the Habsburg monarchy is much too often underestimated and prey to tropes and clichès, even in very good timelines.
This stems, IMHO opinion from the fact that most people who write CP victory TLs are either from the anglosphere, where they usually resent Austria for being "illiberal" or "backward", or from Germany, and German posters of CP victory TLs can't usually do without cherishing the idea of GrossDeutschland.
I've seen Austria or Austria-Hungary do plainly idiotic things in otherwise excellent TLs just because the poster was deeply convinced of the austrians being "stupid" and being systematically ruled by incompetent idiots.
That and, oh, Austria-Hungary systematically loses every war they fight, against anyone, anytime, often by virtue of decisions so illogical as to make a good showing in a nonsensical sit-com.
That said, concerning annexations, as
@Tom_B put It, I Just see the Austrians take the Bor copper mine, which sits right across the border
Ironically, Austria presented WW1 as a defensive war internally in spite of being the ones to trigger it. While there were voices favouring annexations, by and large they seemed resolved not to use the opportunity to expand and instead favoured setting up puppet regimes (in Serbia and Montenegro).
So, a white peace would be very much spun as a victory, particularly if Serbia is defanged (and nobody is going to care overmuch about it after Russia bowed out).
The nationalists are, as of mid-1918 still set in their demands for autonomy and that is a dangerous minefield that could blow Austria's legs out from under it. There are pro-independence groups, but they're, as yet, not the strongest. And most of the pro-independence national councils were in France at the time and reliant on French support, excepting the Czechoslovak Legion (who are stuck in Russia proper). There's also the issue of the returning troops - the Hungarian revolution didn't come out of nowhere and it can trace its roots to returning PoWs who were exposed to communist ideology in Russia. Hungary, with its restrictive voting franchise dominated by landed nobles proved a fertile ground for communist ideology.
In short, I wouldn't underestimate the durability of the Habsburg monarchy, though the possibility of collapse certainly exists.