We report on a powerful mid-IR diode-side-pumped tunable Er:LiYF4 (Er:YLF) laser electro-optically Q-switched with the help of a KTiOPO4 crystal. At a 20 Hz repetition rate, the laser pulses with output energy of 82 mJ and 13 ns duration at the wavelength of 2.67 µm are obtained. At higher repetition rates (up to 50 Hz), one can extract up to 20 mJ from the laser cavity. The developed mid-IR laser source demonstrates high peak (up to 6.3 MW) and average (up to 1.7 W) power. Realized wavelength tuning provides access for megawatt-peak power-level nanosecond laser pulses over the 2667-2851 nm wavelength region, which are highly demanded for mid-IR laser systems development and light-matter interaction study in the view of extreme-state creation in liquids and solids, paving the way to novel microprocessing techniques.