Thank you for your explanation! This gives a better picture of the situation. Taking the Estonian and Aland islands first fits within this TL, but it may not be the best route to St. Petersburg.
I wonder what Finland's status will be in a peace deal with Russia if Finland is not invaded.
If Finland is not invaded by the Germans or their allies, Finland's position would be dependent whether by the time of the peace Finland is still under Russian control, or whether the Finns themselves have created a country in a post-Russian power vacuum and declared it independent (like happened IOTL). If the Russians still obviously control Finland, I don't believe that the Germans would have good reason or diplomatic bargaining chips to detach Finland from Russia. Finland is a peripheral area, and if the Germans have some "extra" influence to use in the negotiations, they would rather spend it on getting more land out of the Russian empire in the Baltic area, the Polish areas or in Ukraine than in Finland.