As someone living in Germany, what you're saying is not true. It is correct that protests have to be registered, that's the case for pretty much all developed countries. It is also correct that the organizers are responsible to cover costs. But the police doesn't restrict people from seeing signs, doesn't blare their sirens. They mostly stay on the side while handling the traffic, and intervene if the protest becomes chaotic. In the vast majority of protests they do literally nothing other than being present.

Compared to the US where multiple protesters have been killed on video by ICE this year Germany is a dream of free speech