I'm no expert but I do suffer from all kinds of mild back pain in different positions.
To me, it looks like your wheel is way too low and too far away.
Roll your shoulders back and down, like when standing with good, but relaxed posture.
Then put your arm straight in front of you, hands at the height of your nipples.
Now your wrist should lay at the top of the steering wheel.
Looking at your picture, it's more your finger tips at the top of the wheel with shoulders being rolled slightly forward and your hand at the height of your belly button
This might induce some forward leaning in your upper back that you don't notice while driving?
I drawed in, where I would expect the steering wheel.
I also drawed the angle between legs and back.
I can't sit with 90°. Too much tension in my hips and lower back. I need a tiny bit more. Like 95°-100°.
So I would need the wheel a lot higher and closer and then lean the seat a little bit backwards.
This might cause the need to pivot the whole seat a bit forward to not sit too laid back.
View attachment 725785
The other thing that might be happening is that your feet are too high or too low or too far away or too close. It's not really visible for an outsider.
The important bit is that your upper legs have as much support from the seat, but without the edge pressing too much into them.
If your upper legs are hanging in the air, you'll have too much pressure in your hips and tension in the lower back.
If the upper legs are too low, blood circulation will be an issue and you might get cold feet or something like that.
Do you have enough contact at the lower back? When I sit at 90° with the feet raised, I often lose contact at my lower back and need an extra cushion.
Or maybe you have plenty of contact but it's too hard when braking, simply causing a pressure point?
In summary:
- wheel higher and closer
- recline the seat a tiny bit
- maybe raise the monitor a little bit. Your head should be in the middle or slightly below
- check the pressure at the upper legs. It's difficult to find the right position. You can either raise them and move them further away until your knees are close to being fully straight. This will make your angle flatter, but might lift your legs too much, when not stepping on the pedal.
OR lower the pedals and move them closer. This will be easier for your hip, but might cause a pressure point on the front edge of the seat, when stepping on the pedals.
It depends on your seat and overall position, which is better for you. But try to get as much of your upper legs firmly supported, while opening the hip angle, but without cutting off blood circulation when braking or at full throttle.