Hi SleeplessinDallas,
Welcome back … I haven’t seen you on the forum in quite some time 
I’m sorry to hear that once again you are looking for a new mattress. While two years is more than most of your previous mattresses lasted before you were no longer sleeping well on them … that’s sort of a mixed blessing because 2 years certainly isn’t as long as most people hope to sleep well on a good quality mattress. As I mentioned when you were last on the forum … you’ve certainly had your share of mattress challenges over the years.
if the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) 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. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
If a mattress/topper combination turns out to be a good “match” for you in terms of “comfort”, firmness, and PPP then having a separate topper also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is very likely because the upper layers of a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time (which in your case may be more likely as well). A suitable topper can also help extend the useful life of the mattress underneath it because it will absorb much of the compression forces that can soften or break down the top layers of the mattress underneath it over time.
As you know … Tempurpedic for the most part uses good quality and durable materials in their mattresses and the biggest issue is that they are in a much higher price range than many similar mattresses that use similar quality materials. In other words if price isn’t the most important issue they can make a good quality choice even though I think they are overpriced and don’t consider them to be good value.
Unfortunately Select Foam never did “fix” their ongoing customer service issues and many of the members here experienced too many issues with their customer service and unreasonable delays in exchanges or refunds so in good conscience I could no longer recommend them as being a reliable company.
The online list that includes the better memory foam options I’m aware of is in post #12 here.
Off the top of my head … Dreamfoam and Christeli and Selectabed and Tempflow all offer mattresses that are comparable to the Tempurpedic Supreme and the others should also be able to give you some guidance about which of their models are closest to the Tempurpedic Supreme in terms of firmness as well.
The Flex Supreme uses the following layers and components …
Comfort Layers:
2" Tempur ES Material 4.0lb
1.2" Tempur Response Material 5.3lb
Bottom Upholstery (below comfort layers)
2.5" Poly foam 1.6lb
Support System: Single Sided Sleep Surface
Pocketed coils, individually wrapped
17" gauge steel
T/840 F/1260 Q/1610 K/2070
Full perimeter foam encasement
Twice tempered steel
The 2.5" polyfoam layer is a little bit lower density than I would normally like to see in the upper layers of a mattress (especially for those in higher weight ranges) although with just over 3" of memory foam above it it would be more durable than if it was in the top layer of the mattress. They also don’t mention the foam density of their foam encasement.
I don’t know of any online manufacturer that advertises a mattress that is designed to be comparable to the Tempurpedic Flex Supreme although Nest Bedding makes a two sided memory foam pocket coil hybrid here that has a medium and firm side using different thicknesses of 4 lb memory foam on each side and they have a 100 day trial period with a no cost refund (if you donate the mattress) so it may be worth talking to them to see if they can tell you whether either side would be similar to the Tempurpedic Flex Supreme.
Other than spending very long hours on the site (as usual) … I’m doing very well, thanks
Phoenix