How are pensions taxed if I move to Bulgaria?

That depends on the type of pension and on which state the double taxation treaty between Germany and Bulgaria assigns the taxing right to. Treaties regularly distinguish between statutory social insurance pensions, pensions from public funds and private pensions, and the allocation is not uniform across them. A blanket answer here would be unserious. We name no rate without knowing the particular pension, and we refer the examination to a tax adviser in Germany and one in Bulgaria. One thing applies in every case: without actually giving up the German residence under section 8 of the Fiscal Code, unlimited tax liability continues.