Lightly grease a 3-quart slow cooker with cooking spray. Combine potatoes, white sugar, 1/3 cup butter, 3 tablespoons brown sugar, eggs, vanilla extract, nutmeg, allspice, ginger, and cardamom in a large bowl. Beat using an electric mixer at medium speed until smooth. Add whipping cream and stir well. Pour into the prepared slow cooker. Combine pecans, 3/4 cup brown sugar, flour, and 2 tablespoons butter in a small bowl. Sprinkle over sweet potato mixture. Cover and cook on High, 3 to 4 hours.