Hi andros69,
There are really two approaches you can take. The first is to use the guidance of someone with the knowledge and experience to “fit” you to a mattress in person and can see how you each interact with different firmness levels or materials in each layer. this is generally the most accurate method if you are testing specifically for pressure relief and alignment and spend long enough fully relaxed on each possible choice. The advantage of this is that if the person you are working with is “good” … you don’t have to know a single spec, just how you feel on each mattress 
The second is more typical of an online purchase which is to focus more on “specs” and theory but this can be more difficult and tricky because there are many variables in a mattress and every layer and the different materials in each particular mattress or “build” will affect all the others to some degree so it can be quite difficult to “predict” how each layer may interact with a particular person unless you know every detail of a mattress besides just the foam layers. The better online outlets and manufacturers are quite good (and in some cases remarkably good) at this if you fall into the “averages” or have tested some local mattresses with known specs because they know every detail of their mattresses and have a large database and experience with hundreds or thousands of customers and many of these would have similar circumstances and height/weight/body shapes and sleeping positions to the person they are working with. They would be quite familiar with the “average” needs and preferences of most body types with their particular mattress in other words and could adjust this up or down if your own testing indicated that you were somehow outside of the average.
In general though … “average” base layers of latex would be in the mid 30’s range and “average” comfort layers would be in the high teens to low/mid 20’s. Any transition layers in between would tend to be in between these ranges and would both add to the effective thickness/softness of the comfort layer and function as part of the support layer as well. They have a dual role in other words.
In your case … “typical” would likely be in the range of 3" of 24 - 28 ILD for a comfort layer and high 30’s to low 40’s for the support layers. In the case of your wife … high teens to low 20’s in a comfort layer would be typical with “average” support layers in the mid 30’s or so.
You may also benefit from a side to side split (where each side of the mattress is layered differently but has the same height).
Side sleeping generally indicates a comfort layer in the 3" - 4" range. Her curvier shape and your heavier weight may both indicate that the thicker side of this range could work well. this could either be part of the top layer itself or it could be a slightly softer support layer or transition layer that adds to the effective thickness/softness of the comfort layer. A slightly softer support layer or a transition layer can both “add to” the comfort layer but can also be firm enough to be fully supportive when they get firmer after the initial “soft” compression (that helps the softness of the comfort layer). The only place where you diverge in your most likely needs is that firmer foam may feel softer to you because your larger weight will compress them more. In other words you may do best with say M/F/XF while she may do best with S/M/F (in the case of 3 x 3" layers which is of course not the only alternative).
There’s more weight/height guidelines here and sleeping position guidelines here and the effect of different types of layering and putting the layers together in the overview here and the sub menu pages in that section.
In the case of the RoomandBoard mattresses … they use Dunlop which is firmer than the equivalent in Talalay. The specs for their Natural Latex mattress that they sent to me some time ago are …
[b]6" Medium firm 5.5 pound core with a 2" soft 4.5 topper laminated to one side giving the option of a dual sided mattress.
100% Joma pure crimped wool quilt fill 1.7 oz. on soft side with wide quilt and 1 oz. on firm side with narrow quilt.
100% organic cotton cover certified organic following the Skul International Standards for sustainable textile production.[/b]
This would be a medium firm base layer (88 kg/m3) and a 72 kg/m3 top layer which in Dunlop is “translated” as soft (although this would not be as soft as “soft” Talalay and may suit you well).
Because the comfort layers only are 2" … more of the firmness of the lower layers would “come through” and it may be on the thin/firm side for her or even you for pressure relief but it would at least provide a baseline for testing and it may work with your preference for firmer mattresses. You may need a thinner topper (@ 1" - 2") on top of this if it is too firm for either of you. The left and right side can also be customized with either latex or memory foam in either firm or soft.
It’s always wiser to go a little too firm because a mattress can easily be softened but a mattress that is too soft for good support and alignment is difficult to “firm up” and correct the alignment.
Good quality latex is the most durable of all foams and premature softening or impressions are almost always from the use of low density/quality polyfoam or memory foam in the comfort layers or quilting and/or any thicker layers of fiber (natural or synthetic) in the quilting of the mattress. In the case of polyfoam or memory foam … the impressions are preceded by softening which may make the mattress unsuitable even if there are no impressions when the weight is off the mattress. Some compression of fiber in a mattress (some will compress more than others) is normal but shouldn’t be a major issue if the fiber layer is not too thick.
You have probably seen the list of the members here who specialize in online purchases but if not they are in post #21 here. They have a wide range of different designs, features, and prices but they are all great value. They also provide a good guideline for “value” as well when you are shopping locally.
You may also have other options near you that have high quality and value mattresses that you could test personally depending on where you live. If you haven’t already seen a post with the better options near you, if you let me know your city or zip I’ll be happy to take a look and see if I know of any.
Phoenix