There are so many variables, seat height, lumber support, reclining, steering wheel reach and rake.
If one of these are out for a particular individual then surely that could be a contributing factor rather than its a Golf / BMW / Audi etc problem?
^^this
The problem is occurring whenever op drives his car. I would bet most of these issues occur because a driver hasn’t adjusted his seating correctly, rather than it being a problem of the car. Seats/steering are infinitely adjustable in most of these cars ( lumbar support is the only thing that may be absent) so you should be able to find an appropriately supportive seating position
I would look at this as a solution, rather than a new car.
Incidentally i’ve had a couple of 3 series- had no problems with comfort
I found the golf seats to be absolutely brilliant. I’m pretty fussy about setting up my seat position- never felt the need to tinker with my adjustments from the day I bought it. Very well designed, in my opinion.
I would suggest to op that he gets a physio to watch him sit in his car- maybe even pay a physio to sit with you as you drive.
Mutley posted an excellent post advising on this- mutley please re-post if you can dig that one up.