> Unless the site is only available to German users, you should be able to file a complain with any of EU member state regulators, and not all are so timid as that.

You can, but they will forward that to the applicable authority, which will be the local German one for German sites.

On what do you base this? Facebook is registered in Ireland, yet multiple EU authorities have fined them.

For very large companies that do business in many countries regulators have various concerns (power of the company relative to the country, jurisdiction shopping via choice of headquarters location, etc) that don't apply for the typical case of a German company with a German audience.

On filing complaints and being informed that they were forwarded to the local authority that has jurisdiction.

https://gdpr.eu/article-55-supervisory-authority-competence/ also states Each supervisory authority shall be competent for the performance of the tasks assigned to and the exercise of the powers conferred on it in accordance with this Regulation on the territory of its own Member State, and article 56 adds to that.