Hi chs4000,
Unfortunately there are too many unknowns, variables, and personal preferences involved and too make differences between different people to be able to predict which topper may work best for you based on a formula or specs (either yours or a mattress) and the only way to know whether a mattress/topper combination will work well for you is based on your own personal sleeping experience.
Having said that … post #2 here and the topper guidelines and sources that it links to can help shift the odds in your favor and make the best possible choice that has the highest odds of success. Some of the suggestions in post #2 here may also be helpful and I would rule out anything that is under your mattress or over your mattress as a possible source of the issues you are having before adding a topper. It may be worth trying your mattress on the floor to rule out any issues with the support system under your mattress.
As you mentioned … if you are uncertain about whether any particular topper will work for you then return or exchange policies can become a much more important part of the “value” of your purchase.
Based on your feedback it seems that you aren’t a fan of the slow response nature of memory foam (especially in thicker layers) so it may be worthwhile sticking with latex or other more resilient materials.
Fiber toppers are also much less resilient and “supportive” than foam and they can work well for providing some cushioning under pressure points or changing the “feel” of the sleeping surface but they are not as effective at redistributing pressure along the body surface and maintaining good alignment. They don’t “hold up” the recessed parts of the body (such as the small of the back) as well as a more resilient foam material.
Based on your feedback this would seem to make sense as the next part of a trial and error process although your experience with the Ikea topper does seem to contradict this to some degree as well. I don’t know the firmness of the Ikea topper so there isn’t a way to use it as a reference point. It has 1 5/8" of latex which is similar in thickness to the memory foam topper you are currently using but the tufting would make it firmer so based on your better experience with the two memory foam toppers (especially the one you have now) it may be worthwhile trying a softer latex topper probably somewhere in the range of 14 - 19 ILD and in about the same thickness range as the topper you currently have (about 2" or a little less).
Phoenix