Please allow me to add some experience at an angle, so to speak.
I've kept the original seats, but:
I've had a shop look at them where they make hand crafted solutions for handicapped people. I simple asked what they would advise.
The specialist there started with telling me that the chairs were fine in principle - no need to consider any fancy stuff.
He did observe that some steel links in the driver seat were broken, so that was an easy welding repair.
The main thing was, he added small inflatable/deflatable cushions at the lower back of both chairs, just above the hips. They each consist of three interconnected air bodies, plus a hand sized bellow to inflate te cushion and a push valve to let the air out. We are therefore able to adjust (while driving) for fatigue, individual preference etc. The interconnected air bodies let the air find the right spot where the back is most hollow.
Last year we've made a journey of over 10.000 km travelling on and off road, and the chairs are excellent as they are now.
My wife's back pain had vanished from the first hour driving and has never returned.
Paid just €500 for the job.
In total.