Hi Ginger,
A few thoughts to add to the mix …
First a few posts that may provide some insights into what may be happening.
Post #2 here has some information that may be helpful in assessing some of the symptoms you are experiencing
Post #8 here has some information about choosing a topper.
I think that part of the problem may be that the PLB toppers are too soft for you. This means that if you use a topper that is too thin that you may feel pressure on your side because you are feeling too much of the firmer layers in the deeper part of the mattress but if you use a topper that is thick enough to isolate you from the firmness of the deeper layers you can have alignment issues because your hips and pelvis can sink down too far relative to the other parts of your body. A mattress that is too firm can lead to upper back issues as well but if you add a topper that is too thick and soft it can also lead to upper back issues on your back because you can end up sleeping in a hunched position which can tense the muscles when they try to compensate and lead to upper back pain or discomfort. The goal is always “just enough” thickness and softness over your support layers.
I think you would do better with a little firmer topper in the 19 - 24 ILD range as was suggested to you and in the range of 2" - 3". If you decide to go thicker (3") then I would consider using the firmer end of the range to compensate for the extra thickness. A 2" or 3" topper plus the 1" 19 ILD comfort layer on top of your mattress would give you 3" to 4" of softer latex which would be more in the normal range for side sleepers.
The goal is to use the thinnest and firmest possible topper that will relieve pressure on your side so that there is a lower risk of alignment issues on your back.
If you feel pressure on your shoulders when you are on your side I would make sure your pillow is thick enough because a pillow that is too thin can increase shoulder pressure. You can also try putting a pillow under your bottom arm from your elbow to your hand as this may also help relieve shoulder pressure.
Phoenix