thank you for your understanding and for respecting nature and the law!
There's always a lot of pressure on nature around big cities. That's why excessive collection of mushrooms, wood, flowers etc. is more of a problem here than elsewhere. Even though most people are reasonable, a small number of people can do a lot of damage to the forest. All the trash is one of the symptoms. Frankfurt citicens have an intimate relation to their forest. At least it the only city I know of that used to have a holyday (Wäldchestag) dedicated to it.
You wouldn't get a permit to collect any Sphagnum unless you are doing serious bryological research and have an affiliation with a university. There are not many places in the forest around Frankfurt where you would find a lot of Sphagnum. You were lucky to find such a spot. It should be left alone. Especially if you can't tell apart rare species from common species. The ground water has dropped since the city needs more and more water. Therefore the wetlands around there are struggling for survival. There used to be places where you could find even Pinguicula vulgaris, Drosera anglica and Utricularia bremii, but they are all gone now.
Growing CPs can be one of the cheapest hobbies you can imagine unless you want to build and run a highland terrarium with demanding plants, lights and cooling. Trade plants with others and all you need is a green thumb, a bright window sill, access to rain water, and some peat, coco coir and/or pine bark.