Thats a very winnable war, build troops I would even go over support limit as this could be an important war.
I would aim for the force release of Bohemia and Tirol, when bohemia gets released they will only have 20% legitmacy, you can then - insult, dissolve alliance, use spy for obscure documents and PU them.
This will give you cores on all the czech provinces and the 2 saxon(cultural assilimation might have happened) can be removed from the empire, Tirol can be diplo vassilised.
edit
Outsiege the AI and have a decent army with your best general shadow the main austrian army, encourage the AI to launch an assault and then attack with the main army and bring up reinforcements.
It might be worth using a spy to infiltrate their admistration which clears the fog of war over their provinces, but since they are emperor they can move any where in the HRE without military access
You will have hurt austria and they will probably try to take tirol in 5 years so be prepared but if bohemia is still around it will be real easy.
another edit
If you try to force PU bohemia the emperor will probably help them as you are not part of the HRE so they will be called to arms, if they have no troops they will probably dishonour the call. You want to be part of the HRE to expand in it as all inheritances in the HRE give cores on HRE territory.
Best way to do this is become emperor and add your capital to it, vassilising 3 electors is the best route to been emperor.
Koln, Trier and mainz are the best since they can't be PU'd unless they have been force released as kingdoms