You can only do some occasional work without registering yourself in any special form.
You can check, if you can start as a Freiberufler. It is a bit more lightweight than going with Einzelunternehmen. For example, I am a Freiberufler now working as a consultant / contractor in data engineering and management. I can also potentially sell my product, but I did not check the limitations.
You do not have to close you Einzelunternehmen afaik, if you do not make money, but you are still responsible for sending regular declarations to the Finanzamt.
You can check, if you can start as a Freiberufler. It is a bit more lightweight than going with Einzelunternehmen. For example, I am a Freiberufler now working as a consultant / contractor in data engineering and management. I can also potentially sell my product, but I did not check the limitations.
You do not have to close you Einzelunternehmen afaik, if you do not make money, but you are still responsible for sending regular declarations to the Finanzamt.