I watch commercials with cool voiceover
https://unimedia.io/voices/canadian-english and have even experimented on the subject myself. You need to remove the voice clamp. Causes of voice stiffness can be different: from psychological clamping to the elementary habit of many years. It is important to loosen the larynx. Watch how you speak - do you use only the throat or do you also use the thorax and abdomen? Use the abdominal area when "speaking" - it's the highest skill, which is taught all vocalists and actors. But not everyone is good at it.