I applaud your recognition that your furniture must be functional, and therefore comfortable ( ergonomically sound).
But don’t be married to your design before you’ve prototyped it. Crisp lines are getting a bit passe anyway.
I’ve sat on a metal seat (40/4 stacker) that was more comfortable than some upholstered pieces. Form is a strong determinant for comfort and an extremely efficient methodology for material usage.
But I also understand a commitment to a design. It sometimes pushes us to find solutions that might have gone undiscovered.
Sorry for the theoretical response, I know you were searching for durometers, etc…