Making the auto-hidden menu bar show up faster

Look for INSPIRE. INSPIRE is a European initiative that nudges member countries to make geographically oriented data available for all, for free.

In the Netherlands this has resulted in -among others- a site PDOK that offers map-layers of all kind. Many of the layers are instantiated from databases and can be queried. Many of these layers have a temporal dimension. One of the layers (in the Netherlands) shows administrative boundaries.

I believe that in Germany Geoportal is one of the providers of INSPIRE data. My knowledge of the German language is insufficient to determine if the exact layers (and temporal extent) you need are available.