Hi dean3326,
I think for the majority of people 3 weeks would be enough time to adjust to a new sleeping surface (most are less than this and a few are more depending on many factors including how used your body is to a different type of mattress) and make a reasonable assessment about how close they are to their “ideal”.
The two “parts” to choosing a topper is to first choose the material you prefer (and would be based on preference and on what you are familiar and comfortable with) and the second is to choose the thickness and the softness or type of the material (or in the case of memory foam the type and density of the memory foam).
Since subjective factors play such an important role in the choice of topper and because there is no way to know for certain which combination will be best for you until you have slept on the combination … post #8 here has some guidelines that are more “subjective” and should help in your choice.
Because you are looking for softer … I would also consider a 4 lb memory foam which in general tend to be a little softer in most people’s perceptions than 5 lb memory foam.
Depending on the more subjective “degree” of softening you are looking for and because there are already fairly thick layers of softer materials in your mattress … I would probably stay in the 1" - 2" range so you are not risking support/alignment more than necessary. Thinner will be less “risky” while thicker will have more of a softening effect.
Phoenix