Hi Jlsgreenwich,
Unfortunately a topper will only be a partial or temporary solution at best for a mattress that has softened or degraded because it will just follow the soft spots or the dips in your mattress which appears to have reached the end of its useful life … at least in terms of providing your needs and preferences.
In some cases … if the comfort layers have softened and you are “going through” them to the firmer layers below them and experiencing pressure relief issues then a topper may help but it also carries the risk of causing alignment issues because by adding softer layers with a topper then the soft layers that are already in your mattress (that have softened) that are designed to provide pressure relief are becoming more “support layers” (the deeper layers in a mattress that are primarily responsible for keeping you in alignment) and they are too soft for this and can allow the heavier parts of your body to sink down too far.
Having said that … if the main issues are pressure relief then a topper can relieve this as long as you are OK with the risk of discomfort or back pain that can come from a mattress where the combined upper layers are too thick/soft and that doesn’t keep you in good alignment.
There is more about this in post #4 here and post #4 here has some suggestions that may help improve support at least as a temporary and/or partial solution.
As far as which type of topper may be best for you it really is a matter of preference. While latex is certainly a more resilient and springy material than memory foam … the instability you are referring to is probably a side effect of the thickness of the topper and the layers it was on top of in your mattress and you would probably have done better if it was over firmer foams. Memory foam is a more energy absorbing material than latex so even though I don’t have high hopes of a real solution in your case with a topper … it may suit you better to use it in this case. Post #4 here includes various sources for toppers of many different kinds and if you are looking at memory foam (for it’s energy absorption and “stability”) then I would look at some of the better gel foams or more breathable memory foam materials. the vendors will tell you about the response and breathability of the memory foams they carry (there are many different types of memory foam).
I would also tend to use a topper that was as thin as you could get away with (2" is generally a reasonable choice depending on the layers on top of your mattress) because the thicker the topper is the more risk you have of alignment and back issues. So much of this depends on knowing the real cause of the issues you are facing and what is already in your mattress that some trial and error may be necessary (which can also be costly if there are no return privileges on a topper).
The Talalay GL slow recovery topper is a little firmer than most memory foam and could still carry the risk of alignment issues IMO on a mattress that has softened or degraded.
So while overall I’m not that hopeful that a topper is the best solution for you and that your needs may be best served with a new mattress … if you do decide that improving pressure relief is worth the risk of alignment issues then for you a thinner less temperature sensitive and more breathable memory foam topper or the Talalay GL (check the thickness) may be your best choice based on your preferences.
A shredded latex topper may also help because it will both displace and compress under your shoulders while it will mainly compress under your hips which have more surface area (holding them up higher) so it would have less risk of alignment issues. Examples are the Seven Comforts here (only available in Queen apparently) and a softer less dense LaNoodles topper here.
If you need to experiment with toppers … then a retailer that allows a refund on their products (which is less common with toppers) including the the Big Box Stores such as WalMart, Costco, and Sams club may be worth considering even though it may involve some time and effort to track down the quality or details of some of their products if they don’t have an accurate description (such as foam density).
Phoenix