Saxony, ruled by King Augustus III of Poland, Was allied to France historically at the start of the War of Austrian Succession. Poland of course contributed nothing to the war because it was Post-Deluge Poland and both sides could easily find someone in the Polish Sejm to veto military action. That's unlikely to change without a major PoD well before the beginning of the war.
Saxony then switched sides following the Hilariously Crushing Defeat of the Franco-Bavarian Armies in Germany which saw Bavaria completely occupied by Austria. Austria then went on to push France entirely from Germany, and it took Prussia re-entering the war on France's Side to actually turn the war back against Austria.
If Prussia is actually allied to Austria then Austria has almost assuredly won the war, probably decisively. And I mean DECISIVELY. I'm talking "The Holy Roman Empire is now Centralized and Austria will probably be retaking territory in Alsace from France" levels of Decisively here. Unless Russia and Britain are both outright supporting France from the start, the Austria-Prussia Combination is simply far stronger than France and a few German Medium Powers. Austria was able to kick the French and their Allies out of their territory despite having to guard against and fight Prussia at the same time. If Prussia is now fighting on Austria's Side and has been from the start, then this is even more hilariously 1-sided. Even if Prussian Involvement replaces British Involvement that's a fine trade as it not only gives Austria a very good army on their side, but removes their most pressing foe.
Poland won't contribute. They can't, their government won't allow it as it is too easily bought off. It didn't contribute historically despite Augustus III being allied to both sides at different parts of the war. Even when Prussia occupied Saxony Poland didn't do anything.
In addition you have to remember that at this time Prussia isn't a Great Power, they're just a Medium German State like Bavaria or Saxony, or at least that is how they are seen, and thus everyone isn't going to look at this Marriage and immediately Panic and consider it The End of The World. This isn't France inheriting Spain, this is Austria obtaining Naples-Sicily.
Other countries probably won't even think Austria-Prussia is a match for France-Bavaria at the start, so you'll probably have a good portion of the early war where Britain is either Neutral, or actively still supporting Austria in an effort to keep The Southern Netherlands out of France's Hands and only realizes the Monster they've helped create later on.
Its entirely possible and likely that the Prussian performance in this war still changes that, and changes it fast enough for people to intervene, but if that intervention happens after Austria-Prussia have already Decisively beaten all of the other German Minors that allied with France and pushed French Armies back into France, that's probably too little too late. At this point Austria-Prussia can do basically whatever they want in Germany and no one is really in a position to stop them. Even France+Russia I think is unlikely to be able to beat them. Britain intervening to help France just gets Hannover Occupied as well, giving Austria-Prussia complete and undisputed control of all of Germany.
Hence "The Holy Roman Empire is now Centralized and Austria will probably be retaking territory in Alsace from France".