Hi geearle,
Infortunately you are in a difficult position of having to choose between mattresses that to some degree all share the same weaknesses or have relatively poor value but there are certainly some options you have that can help you make the best of difficult circumstances. You are unfortunately not alone in experiencing the more rapid softening of lower quality materials in the comfort layers of a mattress which is the “weak link” of most mattresses and the main reason that a mattress needs to be replaced. You are more fortunate than many other though becuse in many cases the foam softening and the loss of comfort and support that goes with it doesn’t usualy lead to the depth of impressions that is considered to be a defect and in most cases (unlike your own experiences) the majority of warranty issues are denied because the impressions aren’t deep enough or some of the other warranty exclusions (such as a small stain) preclude a warranty claim.
Post #2 here includes links to several forum threads and discussions with other forum members that were in a similar position of having to exchange a major brand mattress or a chain store purchase with very limited good quality options available that you may find helpful in making the best possible choice in difficult circumstances and avoiding more repeats of the same cycle.
Phoenix