Hi handsoap,
While Tempurpedic mattresses are not particularly good “value” because there are many other manufacturers that use the same quality of materials that would be just as durable in lower budget ranges … at least the Rhapsody Luxe you purchased uses good quality and durable materials (see post #3 here for the specs) and there are no lower quality materials or weak links in the mattress relative to your body weight that would compromise it’s durability (see the quality/durability guidelines here).
There will be a break in and adjustment period for any new mattress over the first few weeks or (or sometimes a few months with higher quality/density materials) and it’s very possible that you just needed some time for the mattress to break in and your body to adjust to the feel of a mattress that is different from what you were used to (see post #3 here and post #2 here). If a mattress uses high quality materials then once it has gone through the initial break in period then any further foam softening would be much more gradual over time.
It also wouldn’t be unusual that a mattress that was too soft for stomach sleeping was a good “match” in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) for a back sleeper or a side sleeper because stomach sleepers tend to need a firmer mattress to prevent the heavier parts of their body (hips/pelvis) from sinking down too far and sleeping in a swayback position which can lead to discomfort and pain in the lower back or if your pillow is too thick for stomach sleeping in the neck and upper back as well.
Unfortunately the Beautyrest Alexia Black mattress you exchanged for uses much lower quality/density materials than the Tempurpedic Rhapsody Luxe that would be a weak link in the mattress in terms of durability and would be subject to more rapid foam softening and breakdown than the higher quality materials in the Tempurpedic. If you click “overview” on this page you can see how low the quality/density of the materials in your mattress is even though it’s one of the higher end Simmons mattresses. This is one of the reasons that I generally suggest avoiding the major brands such as Simmons, Sealy, and Serta completely because more rapid foam softening and the loss of comfort and support that goes with it usually results in the need to replace the mattress much too quickly relative to the price you paid even for those that are in more average weight ranges and this would be an even bigger issue for those that are in higher weight ranges. I would also keep in mind that foam softening and the loss of comfort and support isn’t covered by mattress warranties (see post #174 here)
As you discovered your Beautyrest mattress with a topper is a very different design with different components and foam layers than the Tempurpedic Rhapsody so it’s not surprising that it feels and sleeps very different for you.
Unfortunately I don’t have any specific suggestions because the first “rule” of mattress shopping (or topper shopping) is to always remember that you are the only one that can feel what you feel on a mattress (or a mattress/topper combination) and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or topper or combination of materials and components would be the best “match” for you in terms of “comfort” or PPP or how a mattress or mattress/topper combination will “feel” to you based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
While you haven’t provided much in the way of specific “symptoms” you are experiencing … it’s also not really possible to “diagnose” mattress comfort issues on a forum with any certainty any because there are too many unique unknowns and variables involved that can affect how each person sleeps on a mattress in terms of PPP or any “symptoms” they experience … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can make the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing.
If the only issue with your mattress is that it’s too firm and you just need some extra softness and pressure relief then a topper can certainly be an effective solution and while once again I don’t have any specific suggestions … there is more information about choosing a topper in post #2 here and the topper guidelines it links to that can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well.
Assuming that the quality and durability of a topper is suitable for your body type … the choice between different types of toppers would be more of a preference and budget choice than a “better/worse” choice. There is also more about the pros and cons of memory foam vs latex in post #2 here.
It’s unlikely that you would need an extra 6" of softer materials on top of your current mattress and even with an extra firm mattress it’s unlikely that you would need a topper that was thicker than 3" - 4".
If I was in your shoes I would probably return the Tempurpedic topper for a refund and try to choose a topper that was a better “match” for you in terms of PPP in combination with your current mattress but I would also make sure that your next topper purchase also has a good return/exchange policy just in case it also doesn’t turn out as well as you hoped for because the only way to know will be based on your personal experience.
Alternatively I would probably sell the mattress and start all over again to purchase a better quality/value mattress.
I or some of the more knowledgeable members of the site can certainly help you to narrow down your options, help you focus on better quality/value choices that are available to you either locally or online, help you identify any lower quality materials or weak links in a mattress you may be considering, act as a fact check, answer many of the specific questions you may have along the way that don’t involve what you will “feel” on a mattress, and help with “how” to choose but only you can decide which specific mattress, manufacturer, or combination of materials is “best for you” regardless of the name of the manufacturer on the label or whether anyone else has the same criteria or circumstances or would make the same choice.
If you decide to purchase a new mattress then the first place to start is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and more importantly know how and why to avoid the worst ones (such as the major brands).
Two of the most important links in the tutorial that I would especially make sure you read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
There is more about the different ways that one mattress can “match” or approximate another one in post #9 here but your current mattress has such a different design from the Tempurpedic Rhapsody Luxe in terms of its materials and components that it’s very unlikely that you would be able to “approximate it” just by adding a topper or toppers.
Phoenix