While only field testing can determine the exact layering that would work best … some general guidelines will give you a good place to begin.
A good starting point for a comfort layer for a side sleeper is typically about 3" in a softer ILD to “fill in the gaps” under the lumbar area and to distribute pressure. The gaps in a sleeping profile when sleeping on the back is less so a good starting point for a back sleeper is about 2". People who are on the thinner side usually also have gaps that are less (their body is less curvy) and so would tend to need a thinner comfort layer. Those who are lighter would not tend to sink in as far into the comfort layer and would typically need a softer material to create a cradle that conforms to their body shape and distributes pressure.
Because of this … a good starting point for you would seem to be slightly thinner than “normal” in the range of 2" in a comfort layer. I would also tend towards softer rather than firmer (because of your lower weight) so while a “typical” comfort layer would be in the range of 19 - 24 ILD (in latex), I would tend towards the lower end of that range … and possibly even lower (a typical memeory foam has an ILD of 15 or even less). ILD (also called IFD) is a measure of foam softness with lower numbers being softer. Most mattresses that use polyfoam in the comfort layers also have a much lower ILD in the comfort layers (even the firmest ones).
The support layer under the comfort layer is the part that holds up your hip area to keep your spine in alignment. This is the part that needs to be firmer. Normally the best materials here are those that have some softness with initial compression (to help the comfort layers form a pressure relieving cradle) and then get firmer quickly with deeper compression. The lighter the body weight the firmer both the comfort layers and the support layers will feel. A typical support layer (for foam) can be anywhere from about 28 ILD up to 44 ILD. Innersprings are not measured in ILD so in this case they would have names that “translate” to softer, medium, or firmer.
So in your case I would be looking for a mattress with 2-3" of a lower ILD in the comfort layer (tending towards 2" if possible) with a firmer support layer underneath it (I would tend towards at least 32 or higher to make sure your spine is in alignment.
When you are testing mattresses … I would pay particular attention to the differences in feel and pressure relief between thinner comfort layers and thicker comfort layers. Knowing the thickess and firmness of a suitable comfort layer is one of the most important parts of finding your “perfect” mattress.
The tranquil has 2" of latex and an inch of quilted polyfoam in the comfort layer. The quilting in the polfoam compresses it slightly so overall this would be the equivalent of just under 3". The wool will make this feel slightly firmer than the ILD of the latex since wool slightly reduces the ability of the comfort layer to conform to the shape of your body. Overall this would seem to be a good place to start.
The Sovn would be similar as it also has a 3" comfort layer however all of it is talalay latex. It also has wool in the quilting but no foam and I suspect that there may be less wool in this one so the comfort layer may be slightly more conforming. It also has an innerspring for the support core which would have a different “feel” from a foam support core. It uses pocketed coils which is a more conforming and slightly softer feeling innerspring than other types.
When you are testing a mattress for pressure relief … it is important to test it in the “curviest” sleeping position (on the side in your case) and relax on the mattress for several minutes. Slightly “bounce” your hips to test if you feel a “sudden” transition to the support layer underneath or if it is a more gradual transition (which is what you want). If the transition is to “sudden” (like feeling a “board” under the comfort layer) … then this could result in hip pressure over the course of the night. Test also if the point of your shoulder is bearing too much weight on your side or if they are sinking in deeply enough to allow the wider torso to take up some of the weight (which is what you want). If you feel the urge to twist your upper body slightly to take some of the weight off your shoulders, the comfort layer may not be quite thick enough or soft enough and this could result in pain, numbness, or tingling in your shoulders and arms over the course of the night.
Once you are satisfied that the comfort layer on the mattress provides good pressure distribution … then test the same mattress for spinal alignment. This is a matter of completely relaxing in each sleeping position and having someone look at the alignment of your spine in each position to make sure it is in alignment. It should be straight on your side and have a natural “S” curve when on your back (similar to its natural curve when you are standing up straight with good posture). Have someone slide their hand under your waist or small of the back to make sure there are no gaps there and that the hand doesn’t slide under too easily (usually indicating a comfort layer that is too thin). Pay particular attention to making sure that the hips/pelvic area don’t sink in too far (indicating that the support layer under the hips is not firm enough).
When you are doing your testing … it usually a good idea to include mattresses with known constructions even if they are not in your price range as this will help you know the overall construction that works for you. The goal is to determine the overall layering that works best rather than only testing mattresses that you are actually considering purchasing.
Bear in mind too that a tighter ticking (mattress cover) or a thicker material in the quilting (such as wool) may affect how far you sink in to a comfort layer (it makes the comfort layer act as if it is slightly firmer).
If you are testing memory foam … make sure that you lay on the mattress long enough to warm it up with your body heat as memory responds to heat and will feel and respond much differently when it is colder than when you have been lying on it for a while.
Hopefully this will help you to discover the overall mattress construction that works best for you. Once you know this it is relatively simple to choose whether to make your purchase locally or if buying a known construction online will save you enough money or provide significantly better quality to make it worth pursuing this option.
If you focus on testing specifically for pressure relief and spinal alignment and seeing the similarities in layering and construction that provide you with both rather than testing based on overall “comfort” or based on just an overall mattress “rating” of firmness or softness (which can be very misleading), your odds of making a purchase that is suitable for your needs and preferences go up dramatically.
Good luck and feel free to post with any specific questions that come out of your testing.