Hi bartian.
Thank you for the hint of starting with phosphoric acid. As I couldn’t really believe this would be possible, I just called my local pharmacy and to my surprise I can get 1 litre (unfortunately, no smaller packages available) of 96% sulphuric acid (though no phosphoric acid, but this shouldn’t matter) there without any obstacle. I only have to bring my ID as it is a hazardous good (“Gefahrgut” in German).
So I will have to refresh my chemistry knowledge from university to be on the save side and will start some pH-lowering tests with the tab-/rain-water mix I use for my tanks to see the effect.