I had similar thoughts with my seats. New trims but very saggy.
I ended up adding 3 small crossbars (15 X 2mm mild steel straps approx. 340mm long), just ziptied them across the base where they intersected all the springs, means the load is spread across all the springs more evenly, reduced the sag to a point I'm more comfortable with, I also added spacers to the seat mounts but could only fit around 16mm high as no more stud length on mounts. Will replace steering wheel down the track with smaller diameter to give more knee-space & hopefully move wheel closer to dash as well.
cheers...
Steve.