If you want to work on flexibility, find yourself a TPI certified physio.
I went to one a few months ago, it was quite good.
I wouldn’t let him touch my swing, but it showed me what exercises i needed to do to be able to get into the correct position for a solid swing.
I am no lesson guru; I average about 2 every 3 years, but I go and see one of the best in Aus. He works with what I bring to the table and doesn’t work on my flexibility, etc.
You coach someone where they are, not where they should be.