I find the only effective means of managing my lower back pain is to keep moving. Type of chair, car seat, cushions etc. make little difference. If flexibility is a problem I would avoid an SUV. We have a Mitsubishi Pajero Sport. My Mrs wanted it because it has seven seats and she believes a bigger vehicle to be safer. I don't like it because of the fuel consumption and, being less agile than average, the 'step-up' makes entering and exiting the vehicle difficult. However, in your particular case, taking time researching and spending money on a quality custom seat may give you the improvement you seek.