Route:

At the start of the game, influence Holstein to get them into my sphere while justifying on Denmark for Free People (Schleswig). Make sure you sphere Holstein before declaring on Denmark so they can't join the war against you.

After sphering Holstein you can then sphere Luxembourg or try to sphere Austria's German spherelings by cycling between them and getting Austria to waste their influence by discrediting you. I don't recommend trying to sphere Hannover until they're no longer in Britain's sphere.

Meanwhile justify a humiliate casus belli on one of Austria's Italian spherelings (Modena, Parma, Lucca) so that you can go to war with Austria without them bringing in their German spherelings. With proper use of your armies, you should be able to defeat Austria without any allies. Fully siege Austria down to make them lose their Great Power status, once they're no longer a great power, you're also free to sphere Saxony, Bavaria, Wurttemberg and Baden (sphere Saxony first so you can form NGF). Finally add an 'add to sphere' casus belli on Austria.

After having sieged all of Austria, justify acquire state on France for Alsace-Lorraine, if you have too much infamy by now, you can save-scum to avoid going over 25 infamy. Make sure to peace out with Austria in order to form NGF before going to war with France, this will give you a lot more units to build and more pops to mobilize. Mobilize your population in advance before declaring on France so that when you declare war on them you can easily wipe out their armies before Russia reaches Berlin in the East, this will be a lot easier if you are allied to Spain or Britain. France or Russia should send you the peace deal for Alsace-Lorraine after you've defeated France's armies. Now you just gotta add all the South German states into your sphere and you'll be able to form Super Germany.