As Brandenburg don't bother trading until you get your own CoT, go off of taxes and production until then. You are losing a lot of cash on merchants. As Brandenburg your main goals are to take Pommerania and to stab Poland in the back when they are busy with TO, Hungary, Austria, Bohemia, Golden Horde etc. The two provinces in Poland that are part of the HRE you can take for free cores, they are really good provinces, and are totally worth 10 infamy to take them. Poland itself you should look at as an easy way of expanding and building a power base. Once the reformation hits and you can change to Protestant (which you will want for the 10% production bonus if you are going Prussia), you can take Unam Sanctum to take Catholic and Orthodox provinces for 2 bb a piece. Which is good because you can then easily expand into Poland, Lithuania, and Russia for little bad boy, which also gives you a route to colonize the hordes (if you want to go that way). In 5.2 Brandenburg will be a bit easier because you can take the one province from Pommerania for a core, which also gives you land access to Teutonic Order. Once you have 7 provinces (4 starting, 2 from Poland, 1 from Pommerania) you should be able to take Teutonic Order, and start taking their provinces because most of Teutonic Order's provinces are terrible and they are really really poor. Danzig, Warmia, Ostpreusen, are good provinces and required to form Prussia. Kurland, Riga, Samogita are okay, and Kurland/Riga will be free cores if you take them from the Tuetonic Order. Gotland, Osel, Estland, etc. are such terrible provinces that it is barely worth having them because they are so atrocious.
While you war outside of the HRE, you should be on the lookout for forcing PUs with minors in Germany. These will help you expand your power base, and when you inherit it will give you free cores. Many of the minors are required to take if you play on forming Germany at some point. Forming PUs are also a good way to expand your power before you go after Poland or Teutonic Order.
Another thing to look out for is Bohemia. You will get missions to take a province from them, and it is a very good province (I can't remember its name). Bohemia will crush you early, especially if they are the HRE. So don't step on their toes before you are strong enough to take them, at least not without help from Austria, Hungary, Bavaria etc.
sending merchants can be pretty expensive, no reason to do it if they get kicked out.
