Hi Underjakk,
Being from the Netherlands I have tried to make a Beerenburg and I think I can at least help you out to get started.
First of all; Beerenburg was originally made by fisherman who bought a mixture of herbs in Amsterdam from a man named Hendrik Beerenburg, they macerated the herbs in Brandy or Genevre while fishing. Nowadays there is a multitude of different spirits that all differ slightly, both named Beerenburg and Berenburg.
The Braumarkt reference you give is for a mixture made by the company Jacob Hooy, they claim to provide the original herbal mixture. Most people that try this one (myself included) feel that is overly bitter after three weeks of maceration. A shorter period of maceration works better. As for proportions; the list of herbs on the package is in order of dosage with the first mentioned herb being the most prominent, this shlould help a little.
This is a different recipe fin Dutch that I haven't tried but is said to give good results, Herbs are very different from the Jacob Hooy package. I have no time to translate at this moment but I guess google translate will do a quicker job anyway;
https://www.stookforum.nl/thread/beerenburger-kruidenbitter-recept/
Roelof