Generally, stretching alone won’t improve flexibility and range-of-motion. In addition to stretching, we recommend practicing Self-Myofascial Release (SMR).
Think of SMR as a self-massage. It can be preformed with a foam roller, trigger point ball (or lacrosse/tennis/softball), or percussion massager (Theragun).
SMR will help you get more out of your stretching and get rid of your tightness more effectively. Try out the following technique to improve your hip mobility for better lower body rotation in your swing.