Hi kinoc,
I know that your mattress may “feel” like the floor and a firm mattress is generally a much better candidate for a topper than a soft one … but I can assure you that there is some soft foam in the upper layers of your mattress that are very different from the firmness of the floor and will affect how a topper/mattress combination performs and even the deeper layers will make a difference as well. There are no specific answers to your questions because each person can be very different but the post I linked will help you make the best possible decisions.
Each person’s definition of soft and firm can also be very different depending on may factors including body type and sleeping style. There are also different types of softness and firmness as well that different people will be more or less sensitive to so when you are looking for a topper it’s best to think in relative and more practical terms rather than 'theoretical" terms that can take into account your own unique perceptions.
It would be a little bit like asking how much salt is the 'best" amount to use in food which would depend on each person’s preferences, the type and amount of food they are adding it to, and what has already been added to the food.
By itself … a topper would probably be too thin to provide either the pressure relief or alignment that most people would be looking for on a floor and need something underneath them (which of course is why they are not used by themselves as a mattress). It’s the combination of materials, components, and design that provides each person with the PPP (Pressure relief, Posture and alignment, and Personal preferences) that they will do best with, not any layer or component by itself. Every layer and component of a sleeping system will affect every other layer (and different people) to differing degrees
Phoenix