Quite surprised you're suffering. My parents bought a 2013 Fabia RS after taking an extended test drive in mine. They've ditched their "lumbar sausages" but have taken quite a time playing with the seat height and rake, and the steering wheel height and reach.

For the steering wheel height don't expect to see all of the speedo, find someone local with VCDS who can enable the large digital speed display instead.

The Fabia has a bit of a sit up and beg driving position because the seats are mounted very high.

Lack of lumbar supports probably dont help either. That said I have lumbar supports in my Mk3 Octavia and they arent particularly effective...also lack lateral support. That thing is doing my back no favours so youre not alone!