You may have the saddle as far back as it will go but maybe it needs to go back even more, I say this because I found I needed another seat post with more setback . From the center line of the seat post to the center of the clamping area is now 25mm compared to the previous distance of 15mm. Maybe something similarly simple will work for you. I second core exercises for any athletic activity and especially cycling.

All muscular activity is connected to the largest bone on the body, either directly or indirectly, namely the pelvis. I have a book on yoga anatomy with very good illustrations and explanations how it is all connected. There is a ton of info on the web about this and one could spend some time profitably reading up on this. Just in case you wish to learn more: https://www.secondsale.com/i/yoga-an...iABEgKVuPD_BwE
You don't have to be able to touch the back of your head to profit from the book.