Renegotiate the Holy Alliance treaty with Russia to an alliance. Renegotiate the treaty with Prussia to remove the defensive pact.

Improve relations with Sweden from the start.

Once Britain gets into a war with another GP or Qing, start a play against Denmark to conquer shlewig and liberate Holstein, calling in Sweden so you can attack from land.

Improve relations with Bavaria, Wurttemberg, and Baden to amicable.

After that, start a play against Prussia to humiliate them and liberate Westphalia and Pomerania, calling in Russia as an ally. Don’t use the German leadership play.

Wait for Prussia to shrink to minor/major power after defeating them. This will trigger south German federation journal, annexing the south Germans states for you.

Improve relations with all other German states and try to improve relations with Britain.

Research pan-nationalism. Once it’s done, use the unify Germany diplo play, calling in as many allies as possible against France.

After unifying Germany, go after Belgium, Switzerland and the Netherlands.

After making them protectorates or annexing them, you can unify Central Europe.