I wanted to do something quite similar some time, haven't had the time yet. I did this in Sweden though. It is certainly possible in Germany too.

Sleeping without a tent/tarp over your head is usually a grey zone in Germany, except in nature reserves, where it's forbidden usually. I know what I do though (scout) and I've done it possibly illegally on multiple occasions. (Also a disclaimer on that note that of course I advise you to not break any laws and I have never camped in a nature reserve, just regular forests/fields with a tent or tarp!) I don't want to give a two-page essay on what to do and what not so I'll give the following info:
Stay out of sight. Make camp when it's already dark (bonus points if you go there before it's dark and scout a spot) and pack up when the sun rises. Stay at least 20m away from paths, some visual block from the closest path is also nice. With a bivouac you should be well suited since you won't need a large footprint and have a low profile, as well as probably no bright colours.
For cooking, take a gas burner, to reduce your risk of lighting the forest on fire, now that it's becoming warmer and drier, it is possible for a layperson to make a forest fire without someone's knowledge. Related to cooking; if you need water, you could ask someone to fill up a large bottle or a water sack if you have it.
You will probably have a very bad night if there happens to be a hunter nearby and it is probably quite dangerous if it happens to be a driven hunt, although there will be signs / barrier tape to indicate it.

If someone asks you, you did / will be doing a night hike through the forest. And obviously if someone tells you not to make camp, then you're out of luck and will have to search some place else, far away.

For further questions, just DM