Welcome! Your truck should automatically use the engine to hold a steady speed going downhill. I drive a steep long grade downhill leaving my house and once I get to 25 (the limit) I hold the brake a bit to stay there then let off. The truck does the remaining 3/4 mile at 25 all by itself. I wouldn’t downshift to brake down from any speed. Very old school. That’s what brakes are for. But I do like how the truck will automatically hold the speed. This may be something you need to enable in settings. I vaguely recall enabling or at least seeing a setting.