Side sleeping generally does best with a thicker softer comfort layer in the 3-4" range depending on weight, body shape, type of foam used, and preferences. This is needed to relieve pressure on the bony prominences such as the hips, pelvic crest, and shoulders. Since side sleeping has a curvier profile … this thicker layer is also needed to “fill in” the gap in the waist/lumbar area.
Back sleepers generally do best with slightly thinner comfort layers in the range of 2-3". This is a less curvy profile and the “pressure points” are not as pronounced but there is still a need for a thick enough layer to fill in the small of the back and the upper thighs.
Stomach sleepers generally do best with the thinnest possible comfort layers which are just thick enough to provide some cushioning for the pelvic bones and other pressure sensitive areas. Too much thickness in the comfort layer in this position can easily lead to the pelvis sinking in too deeply and sleeping in a “hammock” position which can very easily lead to lower back issues. In this position (especially if it is the main position) … it is usually important too to sleep with a pillow under the pelvis/lower abdomen area and to use a much flatter pillow or no pillow at all. This can help offset the effects of this sleeping position.
Combination sleepers generally do best with the thinnest and/or firmest comfort layer which relieves pressure points in their curviest and most pressure prone positions (side sleeping) which will help lower the risk of spinal alignment issues in the less curvy positions (back and stomach sleeping).
The support layers under the comfort layers can either be slightly firmer “transition” layers which can help a comfort layer which is a little on the thin side while still being firm enough with deeper compression to hold up the heavier body parts which tend too sink in too deeply or if the comfort layer is both thick and soft enough to relieve pressure on its own, then a much firmer middle and lower layer (or single layer) is usually best to help hold up the heavier parts.
The lighter someone is, the firmer a particular foam rating will feel to most people. Sensitivity to pressure also plays a role here.
So for your specific circumstances where both of you spend some time on your sides but also in other positions … the comfort layer should be as thin as possible (probably in the range of 3" or so and not “ultra soft”) to accommodate back sleeping. For stomach sleeping even this may be too thick and your choice would depend on how much time you spend on your stomach in relation to other positions. A good choice for many people in this position is a thinner comfort layer (say 2") with a slightly softer middle or transition layer than otherwise would be chosen (still firmer than the comfort layer though) which can help the comfort layer with pressure relief but will also provide better support for back and side sleeping. The quilting on the mattress can also make a difference in how the layers underneath it perform and how deeply they compress.
In the iComfort line, the upper 3 models (or the new top end renewal refined model) have comfort layers that are much too thick IMO to accommodate stomach sleeping or even back sleeping for some in terms of spinal alignment. While I understand that the thicker softer layers may feel really nice in the store, this “initial impression” type of comfort is not the type of long term comfort that will feel good over longer term use as the foam softens and even without the tendency of memory foam and polyfoam to soften, what feels comfortable over the course of a whole night or many nights can be very different from what is comfortable in a few minutes to an hour in the store. There would be a risk of misalignment of the spine … especially with memory foam that tends to allow the heavier areas of the body to sink deeper over the course of the night (memory foam in addition to its other properties has a property called creep which makes it act softer over time)and lead to the risk of back pain.
While you didn’t mention the particular iComfort model that you preferred, the fact that you like the feel of the iComfort points to liking a softer comfort layer and the feel of sinking in to the mattress more than lying on it. Based on this … and subject to your own personal testing confirming this … in a latex mattress (or at least latex in the comfort layer) I would tend to a 3" layer with a firmer transition or support layer or a 2" layer with a more medium support layer. In the second case especially but even in the first case of 3", … the quality of the foam underneath the latex will play a much larger role as a higher quality foam like latex can help keep you in alignment over a greater range of sleeping positions than even high quality polyfoam.
I would certainly be focusing on the thinnest possible comfort layer that relieves the pressure on your side. You are also correct that latex is a much longer lasting material than even high quality and density memory foam which in turn is longer lasting than lower quality and density memory foam. It also doesn’t soften and change its feel and properties over time nearly as much as either memory foam or polyfoam.
Hope this helps and feel free to post more questions. The most important thing I would do is to test various latex comfort layers in the stores to get a sense of the different comfort layers and how they affect you in terms of pressure relief and spinal alignment. These can serve as a model for an online purchase as latex can come in different thicknesses, different ILD’s (softness firmness ratings) and with different layers underneath it … all of which can help “point to” the choices that are best for you in an online purchase.