I has the S3 19 and then upgraded to the 21. I did not notice a difference. However, I have a healthly back. I would think that you could depower the kite a bit to lessen the load on the harness. It is possible that the design tweaks to the 21 might increase the harness load. I would ask this question on www.foilzone.com
If you have access to a 19m from a friend, it would be helpful to see if it the kite or if the condition of your back has deteriorated.
I started doing Pilates a few years ago and it has done wonders for my back. If you can find a teacher that specializes in Pilates Therapy, that is a good way to strengthen your back.