The attempt to Germanize Hungary and the assimilation of the Jews need to be more successful. Austria should not participate in the Partitions of Poland or conquer any more Ottoman territory after 1700 because this would only dilute the overall German element.
After 1700 a lot of Germans went to the Americas, Russia, Poland, Britain, eastern Germany, Australia, and the German colonies. Find a way to get these settlers to go to Austria.
AH was one of major "importers" of German migrants in 18th century especially to Hungary, it only stopped with chaos in the Napoleons Wars, just as the migration to USA more or less stopped in that periode*.
So we need continued immigration, no annexion of Galicia, and no Napoleons Wars.
Maybe something like Kabraloths POD in this thread.
https://www.alternatehistory.com/discussion/showthread.php?t=83010
This could sabotage the Polish partipation.
Beside it would give Austria territorium which it could later trade for Bavaria** together with Flanders and Luxembourg. That would give the Joseph II the prestige needed to reform the Empire by incorporate Hungary and Croatia directly in Austrian Empire***, that together with the removal of Serfhood**** (in Hungary) would result in a economical boom both in Hungary and Austria, which could result in greater immigration. Beside that Josephs hostility toward the Catholic Church could help intergrating the Orthodocs population in the empire.
The effect of a won Seven Years War for France could also butterfly both ARW and the French Revolution away, so no Napoleons Wars. Which push the birth of modern (Romantic) Nationalism at least a few decades out in the future and keep German emigration going. Which give Austria time to force French style Nationalism down over the empire.
*and was only started again around 1840.
**Which is important because it rid Austria of Flanders and Luxembourg and help Austria getting out of Germany, by creating a strong Wittelbach state in the West.
***Maybe with a small civil war which crush the Hungarian elite, so they can be replaced with people loyal to Vienna.
****Which was one of the other thing the Hungarian nobles rebelled against.