I too have suffered from back pain over the years, although mine is lower back.
The best standard seat I've used was the one in my Mitsubishi Cordia.
It wrapped around enough to give good support while cornering, without being restrictive and had adjustable lumbar support.
The cushions were firm without being too hard and being a two door car, the seats fold forward.
I agree about the overall vehicle ride being important.
I don't run my tyres at max side wall pressures for comfort reasons. Instead, I split the difference between side wall and recommended pressure as a compromise between FE and not feeling every ripple on the road.
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