essentially braking when sim racing isn't the same as a real car without the forward force as you slow rapidly. instead you're shoving your back into the chair.
I have a broken back and have been having this issue with the RS9 chair on my GT Omega Prime. That chair sucks. Ive had to add a memory foam back pad and extra lumbar support to reduce my back pain. But for the amount of damage I have some pain will be inevitable for me.
maybe add a back pad and adjust your positions so when you brake hard its pushing you back in a way that hurts the least.
here's what I bought: https://www.amazon.com/dp/B0943STWRQ?psc=1&ref=ppx_yo2_dt_b_product_details
then I have another smaller pad underneath that I can adjust for extra support where I need it if I have pain.