Hi JannyGoats,
Numb arms usually means that the mattress doesn’t have enough thickness/softness to relieve pressure when you are sleeping on your side. This would usually indicate the need for more thickness softness in the comfort layers.
If you could describe the specifics of why the 19 ILD topper didn’t work for you and the specific symptoms you were experiencing it may be useful as a reference point. You may have gone from one end of the range to the other and “jumped over” the thickness and softness you needed.
In scanning your previous posts it seemed that you were a side sleeper and occasional back sleeper but from this comment it seems that you also spend time on your stomach which generally needs thinner/firmer comfort layers to prevent the pelvis from sinking down too far which results in hyperextension of the lumbar and a swayback position which can lead to lower back pain. If you are a combination side/stomach sleeper then it’s usually a good idea to have “just barely enough” thickness and softness in the comfort layers to relieve pressure so that there is less risk of alignment issues when you are sleeping on your stomach.
Stomach side sleeping is a difficult combination to get right and generally involved experimenting with both thickness and softness. If you use a softer 3" layer it may improve pressure relief but it could make your stomach sleeping symptoms worse. It may also be worth considering a body pillow so you don’t turn on to your stomach and still have the sensation of having something pressed against your stomach which some stomach sleepers need.
Post #2 here and the posts it links to has more information about some of the more common issues and symptoms that may be helpful to assess some of the underlying reasons for any issues you may be having.
Post #2 here and the posts it links to may also be helpful in helping you decide on the thickness and softness of a topper that may work best for you but if you spend time on your stomach then 3" may be thicker than your ideal.
There are hundreds of variations of basic innerspring mattresses that each have different designs so it may be helpful if you knew what the ones you sleep well on had in common.
Phoenix