If artificial reservoirs were feasible, they would be better used to flatten the production from renewables.
Of course, they require the appropriate geographical features, but those features are relatively widely available in hilly landscapes, which are rather abundant in large parts of Germany.
The reasons why relatively few hydroelectric pump storage power plants have been built in Germany in the recent decades are entirely homemade. For once, the spirit of NIMBY is very strong in Germany, so if you’re planning to build something like that, you’ll be facing the wrath of a plethora of angry German Spießer forming citizens’ initiatives and fighting your project. On top of that, there is German bureaucracy, which will ensure, that the volume paperwork you’ll have to file for building your reservoir is sufficient for filling it up, should you happen to drop it in there. Then, there is the privatised power grid and its idiotic circumstances and rules, which make it unlikely for a pump storage power plant to be profitable, but thanks to the ideology of having privatised essential infrastructure, the state isn’t going to operate them.
It’s only the maximum possible due to NIMBYs, misguided self proclaimed “environmentalists”, German bureaucrats and their petty shit preventing a dozen or so of new ones from being built in the past two decades. On top of that, idiotic “energy market” rules make them very unlikely to operate at profit, but thanks to the neoliberal ideology, essential infrastructure can’t be state operated.