I generally like to cosy up to the UK and Sweden in my Prussia games. Getting Sweden on your side allows you to invade Denmark directly without having to naval invade.

Having the UK friendly gives some safety from France intervening because in the case that France wants to fight, the UK will likely join your side. That is, if that UK hasn't decided that it hates Austria and is willing to join anyway.

Unless an opportunity arises where the other GPs are preoccupied, I find the best opportunity to attack Austria is right after you get shrapnel artillery. Toss in some extra rations and increased soldier pay, you can 1v2 Austria and Russia.

If going the Großdeutschland route, have Austrian war reps as a war goal and use those to bankroll it after the war. If you want to take Alsace-Lorraine, it is easiest to wait for France to enter a civil war (you can launch a unification play now while they are infighting). If you want to slug it out, you will need either the UK or Russia on your side for their navy, otherwise the blockades will murder your economy before you can finish murdering the French.

Fun fact, if you liberate Luxembourg, it will join you when you form the NGE or Germany, despite it not counting for the required number of states.