Once again, I suggest we consider Stettin carefully. As you may remember, I spent a few years in the castle garrison there... If you can call it a garrison. If you can call the wooden shack there a castle.

The place is a dump, and for the life of me, I can't imagine any way to make the city particularly profitable or prosperous. Perhaps I just lack the experience to see the opportunities clearly, but the area is not particularly fertile, it is one of our sparsest populated regions, and there aren't even any noteworthy trade resources. The place is useless as a city, and useless for trade purposes. Even if it was converted to a city, and even if we ignored the hopeless trade prospects, it would take eternities for the city to reach even average size.

As far as I can see, the only reasonable use for the place is as a castle. That does not require trade resources, and it does not require a large population. And if Hamburg is converted to a city, we may need nearby sources of good soldiers for quick reinforcements. Training soldiers is one of the only activities that can be done effectively in a deserted mudhole.

However, taking Thorn would definitely be worthwhile.

That leaves the question of Magdeburg. The city has no naval access, which limits its trade potential. But at least it has some trade resources in the region, and it has a much larger population than Stettin. So it would at least be acceptable as a city, even if it won't ever become a goldmine.
But it could also work as a castle, supplying troops to the rest of Franconia. I suspect our financial situation would require a city rather than a castle though, especially if we keep Stettin a castle.