Grounded, ready, engaged, solidly planted, ready for speed. Ready so that no one could push you over. These are expressions of “posture readiness” needed for making a powerful golf swing, feeling properly and solidly balanced from start to finish. Ie Tommy Fleetwood, Brian Harmon.
Bending from the waist is necessary, but not to be over done. Bad backs and injured knees make perfect posture impossible. Being ready, steady and balanced is most important. Your weight in the middle of your shoes. Are you engaged in your set up? (like football linebackers getting set), remember you are going from standing still to a high speed of swing. There is no perfect posture but there is one that fits each individual, a “preference”. The fun of practice is to discover which is best for you. Try reaching, try keeping arms close to you, Try a wide stance a narrow stance and variations of waist bend. It’s all about a solid strike and consistency, “posture readiness“ will help.