I'm having a hard time seeing Frederick Christian do any better than OTL Stanislaw August Poniatowski in this scenario. Actually, he might even do worse.
Stanislaw August was able to push through so many reforms in his reign because he had the Familia on his side, who were pro-reform. Frederick Christian is not only a Wettin, which by then had become almost universally despised through the Commonwealth due to their indifference towards the problems of the state, but he also has no internal faction supporting him. The Familia are against him, the soon-to-be Radom Confederation will be against him, and the only major ally he has in this scenario is Austria, who I doubt would want the Republic of Both Nations to grow any stronger.
ITTL he does have a faction supporting him, the Prussian Faction.
Did you not read the first post? East Prussia was traded to the PLC for Courland and to smooth the transition August III had to give the Junkers seats in the Sejm or face revolt.
What about Austrian Netherlands ITTL? IOTL Austria agreed to exchange Southern Netherlands (which would be given to Philip of Parma) with Duchy of Parma (second Treaty of Versailles) but those plans were abandoned because quick victory over Prussia was not achieved as planned.
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