Hi niroth,
Unfortunately ⌠based on my observations and customer feedback over the years ⌠I donât believe that FBM even sells talalay latex and I also donât believe that the latex that they do sell is soft at all. They seem to think (also based on comments they have made to various people over the years) that if you cut a layer of medium/firm latex in half that it somehow changes the ILD of the thinner layer which is not the case at all. Itâs quite likely that the 1" layer that you have is firmer than the Talalay that you have underneath it. You can see more of my thoughts about FBM in post #2 here and post #2 here. what this means is that you are dealing with an unknown layer and have no way of knowing with any certainty whether adding 1" of soft talalay would have helped you.
The layering you have would generally be good for stomach sleeping (which needs a firmer surface and good deep support to prevent sleeping in a swayback position) but would not generally be thick/soft enough to provide good pressure relief on your side for most side sleepers ⌠especially over firmer coils.
The Talalay you have would either be 28 or 32 ILD if it was blended Talalay (which Latex International calls ânaturalâ) and it would probably be N3 or N4 if it is100% natural Talalay (which LI calls âall naturalâ). You can see the ILD ranges for their âall naturalâ Talalay here. The blended is more pressure relieving and more âexactâ in terms of ILD than the âall naturalâ which only is rated as a range of ILDâs.
So I would use your experience on the mattress with the 2" of Talalay on top in combination with the topper guidelines in post #2 here to choose the additional layer that would suit you best. Based on your description ⌠I would guess that âa little to a fair bitâ of âsoftâ latex (in the range of 19 to 24 ILD) is probably in the range you are looking for which would mean another 2". I would also only purchase from a supplier that you can have some confidence that what you are buying is the same as what is described and if they had a refund policy it would provide you with some assurance that you could exchange it if you guessed wrong about either the thickness or the softness of the layer.
This would likely give you the layers that had good odds of providing you the layering options that you needed to be inside your âidealâ range of pressure relief and support. You would have âŚ
2" of latex that was either 28 ILD, 32 ILD, or N3 or N4 depending on whether it is ânaturalâ or âall naturalâ
2" of soft latex of either 19 or 24 ILD or N2 if you go with all natural
1" of unknown latex that was probably in the medium/firm range for Dunlop latex
3/8" of firmer polyfoam
The foam that is quilted in your cover.
The layers that you removed were likely higher density than 1.6 lbs if they were memory foam but density and firmness are only loosely related with memory foam and are not really related at all if it was polyfoam. In any case these are the layers you removed so it really is a moot point anyway.
The fact that you sleep on your side mostly in response to mattresses that have better pressure relief also points to more thickness/softness being necessary.
So while there is no way to know for certain which layer thickness and softness would work best outside of your own personal experience and some potential trial and error ⌠this at least may help you with making higher probability choices 
If you do choose to go in this direction ⌠then you may still need to experiment a bit with layering and you may not end up using all your layers (such as the unknown foam from FBM or you may put this on the bottom and then remove the 3/8 piece of polyfoam for example or test the layers with and without the cover) but you would at least have some reasonable options that would have better odds of solving the âsymptomsâ that you are experiencing.
Phoenix