These seats and the runners are all one piece/integrated with the OEM seats I believe so your choice would be to buy a seat rail for £120ish plus a recliner bucket that you feel fits you better from about £250 upwards (multiply both costs x 2 if you're looking to keep things matched or as Wilmo suggested a more cost effective solution may be the seats from another non type R 8th Gen. Might even get leather!!
If you were looking to enhance the car I'd have suggested the former option as lower seat rails would address one of the common complaints about the seating position being a bit high and you'd save a fair few KGs too but with back troubles a lower seat position probably isn't something you'd want to entertain.
If you do look at changing seats I'd obviously look after the standard ones as they'll need to go back in for resale or you could struggle to find a buyer.
This back issue from an old injury? If not, perhaps see a chiropractor? My other half used to regularly suffer with sciatica and 4 or 5 of those sessions sorted her out. Amazing really.
It's sounding less like you don't fit the seats and more about the pressure points in your back.
Whatever the case, back pain is miserable as hell so hope you get to a solution that still involves you enjoying ownership :smt023