BTDT... Have had a past back injury too. You didn't mention what year your Z is, but the 90 in particular has high bolsters = adding to the sill issue.
For me, the sport seats were too narrow for my not really so wide a butt (having a 38" waist), and I found a pair of base 95 seats (which I love!) that really make a big difference getting in and out. (The Z sport seat bolsters are incrementally lower on subsequent model years. Sport seats from a 94-95 or GS are probably the best C4 seat option, IMO.)
Other than that, "sit and spin" might work (again, bolsters allowing), ya just have to be able to have the door wide open to do it.
Now that I'm healed, and have the base seats, I can step in with my right foot, turn my body slightly to the left, fold - sit- and pull left foot in, ALL in one motion.
Anyway...Good luck with your recovery.
