I also have severe problems with sciatica that are, thankfully, slowly being resolved by PT. Regardless of the car that I drive, I find that using a lumbar roll is a real life saver. Some cars have supposed “lumbar supports” built in but these don’t do a heck of a lot for me.
I have several McKenzie lumbar rolls around and just keep one permanently affixed to my car seat. If all else fails, a rolled towel works basically the same. I just wedge it in my lower back, just above my hips or belt line.
But my experience is that all car seats are bad, just some are worse than others. I’d rather have a stiff economy car seat than an overly soft luxury seat.