Have possibly figured out the source for the letter: G. Tr. Gallus, Geschichte der Mark Brandenburg. Hopefully one of us can track that down.

Oh, look, it's on archive.org! In that ridiculous font. selenak will you do the honors of telling us what might be of interest in this letter? I can, but it'll be slower and involve more cursing.

I definitely see the "la Mort est douce pour un si aimable Prince" quote, though! Found it!