When I got my '97, it was 16 years old. The driver's seat was soft and squashed down. So I had a local upholstery shop restuff the bottom with a stiff grade of foam. This caused the bolsters on the bottom to become stiff and elevated, which caused cutoff of circulation in my legs after driving an hour or so. Maybe a different type/grade of stiff foam would have been better...but what type?
I recently replaced the custom stiff foam with OEM (PAD, FR SEAT CUSHION,71511-35050) and it is super soft...jelly-like, really. My butt sinks down deep into the cushion, which cuts off the circulation to my legs after awhile. Temp fix: stuff a piece of floor mat under the thin Seat Saver seat cover.
If you like soft seat bottoms, get the OEM foam.
I never worried about the springs in the seat bottom, they look OK...do they get soft also?
All 4 Toyota trucks I've had have lousy seats...particularly the seat bottoms... compared to normal cars. My friend's 2001 Mazda Protege 5 has completely superior seats.