Hi dhaiman1.
Thank you for the additional information which only confirms that you are going in a good direction with fine-tuning your mattress.
Your guess is correct. I always discourage people from making decisions based on a firmness scale. Definitions of soft and firm can vary so much. Comparing the softness/firmness of a mattress you might consider with another mattress on the market no matter how widely available might be is not possible (except in general terms) because a mattress works as a whole and in combination with the individuals sleeping on it.
Any firmness rating scale is unreliable at worst and at best only possible as a vendor’s internal tool to compare mattresses made by the same company or/and … tested by the same individuals. E.g. an “hour-glass”, very curvy body profile, side sleeping sleeper might have trouble with most mattresses even in soft configuration. This 3-page more extreme alignment/pressure points case can give you a good idea of how different the perception of softness/firmness can be from person to person.
Nobody can tell for sure how 2" vs 3" Talalay topper on top of the current mattress will play out for you as you are the only one that can feel what you feel on a mattress and there are many interdependent variables involved (both personal and those of the mattress itself). Generally speaking, the difference between a 2" and a 3" comfort layer can be quite dramatic and also depends on a person’s weight, shape, sleeping positions, and where the weight is carried. Side sleepers will notice it more (they are more likely to go “through” a thinner comfort layer) and feel more of the properties of the layers below it. The thinner a layer is … the more you will feel the properties of the layers below it.
This said 2" 19 ILD topper upon your existing Luma mattress has a fairly high probability of giving you the shoulder pressure relief you need without detracting from the support necessary to keep your spine in neutral alignment during sleep.
Hope this helps … and it seems you’re getting closer.
Phoenix