Prussia can also be helped if France decides instead to throw more troops to North America, remaining more defensive in Europe. Remember, Britain was France's primary target. Also, you can have the traditional Franco-Prussian alliance remain in place. These two things would immensely help Prussia.
Prussia and France would easily overrun Hanover and Brunswick, Prussia could probably accomplish this on its own. France would be left to take on the Austrian Netherlands, which shouldn't be too hard.
Afterwards, French troops assist in a war against Austria and possibly Saxony. Sweden would most likely ally itself to France and Prussia. Russia however, would probably be allied to the British in this TL, and the United Provinces would be drawn in on the Anglo-Austrian side. Denmark is a another country that may join as well, most likely with the Anglo-Austrian-Russian alliance.