Hi NAS23,
Mine has been causing me the same problem for the last 8 months or more, and its driving me crazy.
I thought it was my aging back (I'm 48), then I had a 2019 BMW 330D Estate to roll around in for 6 weeks and that was really comfy.
Then I thought it was my bed casing me problems, but I moved house and stayed with friends, 5 different beds over the same period.
I've tried adjusting it all from one extreme to the other which makes very little difference.
As fredgroves suggests I think it's my sciatic nerve, but its strange that only driving my CS40 affects me so bad, with the problem coming on within 10min of starting to drive and drives over an hour become agony.
In every other part of my life I only get less than 5% of the problem, So I figure the problem is caused by the car and the non-car symptoms are just left over from that.
Like you, I'm not a unusual shape, 5'11" and 86Kg, so pretty average.
It's shame as I LOVE the car!