@MarkHowells; not a Golf, but I had similar problems in my 2020 Polo GTI+. Design-wise, the seats looked pretty much the same as the sports seats VW fitted in the mk7 and mk7.5 Golf GTI so I was expecting them to offer good support and comfort. However, for me the comfort was lacking; it took me many, many months of fine tuning of the seat fore/aft, height and lumbar adjustments, along with steering wheel reach/rake and height adjustment of the upper seat belt mounting point before I found an position that was just about OK (if I was feeling generous, I’d score it a 6.5-7 out of 10). I found even on short journeys I would experience numbness in my lower left back and left leg. I’ve always used the left foot rest in all my cars - whether manual or auto transmission, so I didn’t change my leg to pedal position in the GTI+ compared to previous cars I’ve owned.
I’m a long term bad back sufferer but I never had this issue in any of the previous four VW’s I’ve owned; all were extremely comfortable - especially the sports seats in my mk6 Golf GTD.
Hope you manage to find that ‘sweet spot’ soon.