We would suggest investigating orthopaedic cushions in the first instance, as although you don't feel they are appropriate they will be the most straightforward and cost-effective option. You could look at having the cushioning in your current seat replaced, but this would be a specialist task and likely expensive. Replacing the seat would also be problematic - you could look at other vehicles in the Volkswagen Group on the same platform to see if they have other seating options which would fit, but fitting a seat from a different manufacturer would cause issues with seat belt pre-tensioners and airbags.