[Retroman1969]

I totally feel for you, seriously.
I've lived with mild scoliosis since birth which has resulted in a weak lower back in the onset of middle-age. If not careful, I can land myself in the hospital for weeks. Fortunately rarely happens. Oddly, it doesn't seem to be affected by the seating position of the Boxster. The seats are firm enough with good lateral support and efficient positioning.
Getting in and out while recovering from an "episode" can be challenging, but I just grab the roof and A-pillar for support and turn sideways to get in or out.
By contrast: The tall upright seats of my Jeep don't help in the face of the bouncy ride pummeling my back into hamburger, and the seats of a big sedan are usually too flat and soft to give proper support.
Hopefully you'll be able to find a comfort zone with your Boxster until you have recovered.