Sitting straight up in a sofa, a 30" depth seat would be very uncomfortable unless your over 7ft tall or something? A 27" seat depth is more common for very tall individuals. But if thats what you want then you can custom order a sofa to not have back pillows, instead it will be a tight back with about 2" of foam. Most sofa frames have a seat depth of about 28" to 32" without any padding or back pillows.

Ask about latex foam for long-lasting firmness options. All natural Talalay or Dupont method produced are available in different firmness ratings. They will not deteriorate over time like poly. You will have to figure out the correct firmness(ILD) for seat and back. They'll be different.