Tbh, I think your issue lies with the seat. One piece racing seat are made for cornering, braking and acceleration forces, forces we experience non off. I know this dosent fit your (awesome) rig, but I went with a simple Ikea Poang chair. It, crucially, supports my lumbar area which makes sure you dont restrict bloodflow to your ass and legs. I've been in all kinds of racing seats over the years and while fine for a real racecar, they suck for simracing imo.