Hi rvsarch,
Iâm sorry to learn youâre not enjoying the feel of your memory foam topper. 
As we discussed in one of my replies to you a little over a year ago, it is typical that memory foam will soften over time under the heavier parts of the body especially (virtual impressions) more than sag (visible impressions) and will also lose some of its âmemoryâ.
Assuming the latex core is still in good shape (you can place it directly upon the floor and lie back upon it without the memory foam topper on it) and there is no sag in your foundation, choosing to go with a latex upper 3" topper can be a good alternative, although it will have a different feel from the memory foam that your wife really seems to enjoy.
Latex has an unusual combination of surface softness and deeper firmness/support that comes from its elasticity, itâs point elasticity (ability to conform to the shape of a body) and its compression modulus (the ability to get firmer faster with deeper compression than other types of foam). This means that it can enhance the pressure relieving layers above it because of its surface softness and point elasticity but it is also very supportive and can âstopâ the heavier parts of the body from sinking in too deeply. It is also very resilient (it returns a high percentage of the energy of compression instead of absorbing it like memory foam which has very low resilience) so it can enhance the âfeelâ of the mattress by making it more responsive and more adaptable to different body profiles and sleeping positions rather than the less responsive feeling of memory foam or the stiffer and less adaptable characteristics of polyfoam. Latex is also a very durable material (itâs the most durable of all the foam types) so it can add to the durability of a mattress compared to other materials but the deeper layers of a mattress have less effect on mattress durability (a mattress will soften and break down from the top down) and more of an effect on performance and support/alignment so this would be less of a factor in its use.
All memory foam will tend to be in the very plush category (low to mid-teens for IFD). Thereâs more about selecting a latex topper here, but if you need a fair bit to a lot of extra softness then a 3" topper would probably be the best choice. 19 - 24 ILD in latex would be considered to be in the soft range for âaverageâ body types while very light body types may be OK with an ultra soft 14 ILD and heavier body types would probably do better with 28 ILD and above. If you choose a topper that is âtoo softâ, you can actually end up âfeeling throughâ it to your latex core, especially when placing your weight on your elbows, so the 19-24 ILD range may be a good place for you to start investigating.
Because of the uncertainty involved with purchasing a topper where you canât test the combination in person ⌠a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesnât turn out as well as you hoped for.
Phoenix