Hi mltsao,
I don’t think that the foundation would be the problem in your case and your slats would have roughly the same level of firmness as the floor.
The firmness or softness of a topper will depend on the softness of the material itself, its thickness (thicker acts softer) and on any other materials that are either over or under the topper as well. For example the 2" memory foam topper you used on the floor would be so thin and soft that you would “go through” it and feel the floor underneath it (see post #14 here). It would act differently on top of a mattress because the material under it would be softer than the floor.
In addition to this … there are different “types” of firmness and softness. Thy include surface softness, pressure relief firmness/softness, and support firmness/softness. Each of these has a different function in a mattress and fixing one won’t fix “symptoms” that are caused by the other. Because of the firmness of your mattress though … it appears that you are dealing with not having enough thickness and softness in your comfort layers.
In addition to this … lighter weights will sink into a material less than someone who is heavier which means that mattresses can feel firmer to them than they would for heavier people. All mattresses need to provide you with a balance between primary support (primarily from the deeper layers), secondary support (primarily from the upper layers) and pressure relief (also primarily from the upper layers).
So to 'translate" all of this … it appears that you purchased a Sultan FJORDGARD which is “most firm” and because of your lighter weight it would probably feel even firmer to you. Being a side and back sleeper … very firm mattresses could lead to ether pressure issues and/or alignment issues because there isn’t enough soft material for you to sink into … especially on your side. (see this diagram).
This usually means that a topper would probably be the best solution and for this the the topper guidelines in post #2 here may be helpful.
Phoenix