Kongo cannot build holy districts, but what happens when you capture them? They're not destroyed are they? I have no idea if this works, but how about letting your nearest religious neighbor spread their religion to you, then declare war and kill them, appropriating their shrines and temples and relics? You also get to keep their founder belief. It shouldn't be hard to get another religious civ to join you in a joint war to reduce the warmonger stigma; the ancient era is probably too soon to reenact the Great Betrayal.