There are two problems for fast forming Germany as Brandenburg. Emperor and the unlawful territory infamy penalty. And their missions are very under-powered, don't have any core-giving missions, but the missions require couple of conquests.
Bavaria on the other hand is much wealthier, and has core missions on Franken, Ansbach, Pfalz and Worms. And if you get lucky as I did, you will get "Claims on your rivals" as first event in the game (ofc I saved and play every Bavaria game from that save :wub

Also if you are in HRE you WILL NOT get random conquer minor/neighbour/province mission, and boundary dispute wont fire if the province is capital, which in the case of many HRE minors is...
Best answer is PU's, vassals and vassal annexation missions, or leaving the HRE to get conquest missions. I usually mix the two, using the "leave HRE" once I become powerful enough to hold out on my own.
If you play with Brandenburg, the moment TO turns Prussia, RM them and claim throne, DOW and form PU. If you are playing TO, use the random conquest missions to your advantage. If Bavaria rely on PU, vassalisation. It will probably take more time, but having almost all of southern germany cored gets you invincible. Just Befrien Habsburgs and/or Bohemia with RM, and wait for a time to strike. Austria can be dealt with realeasing Tyrol and Styria and even vassalising/PU's on the two afterwards.
I am playing game with Saxony where I'm trying to form Germany, but it's bloody hard, 10 years into the game, I just sit and wait, I war-forced SaxeLUneburg into PU but thats it, I am forced to sit and wait for something so I can move on. But it will be fun Saxony-> Prussia-> Germany game I hope.