Problem is that Frederick II (of Prussia) was most likely or homosexual or impotent (or both), so yes to a marriage, but no to any descendants and no to a personal union.
Break of Greece (including western Anatolia and Constantinople) and form a Byzantine Empire, ruled in Personal Union by the Tsar who becomes Emperor of the Rhomans. In the east, form Greater Armenia and Greater, Christian Syria (also ruled by the Tsar).
In Victoria II I would strongly advocate a Russian conquest of China
But, even if China was often ruled by foreign dynasties, this is really ASB, as long as the Russian Tsar stays a Russian. To rule China effectivly (and not only some ports like the Europeans did - this underlines their impuissance against whole China), you have to move your capital to Beijing, adopt Chinese culture and basically become a Chinese. And even then they would treat you as a forgeigner for generations.
The Polish were very nationalist in these times. The only solution to the problem I see is liberate them and rule them in Personal Union. Then wage wars against Germany and Austria to enlarge Poland.
Having the Tsar of Russia rule China directly may be insanity, but is the idea of the Russians overrunning China then placing a third son as the Emperor of a China sans Manchuria, Mongolia and Uighurstan as a Russia-aligned state insane in the 'practically unfeasible' department? Difficult to pull off sure, but it wouldn't be the first time China's been ruled by a foreigner.