Hi redmed and welcome the Mattress Underground 
Sorry to hear that your mattress is no longer performing as it once did - although 20+ years is quite a good run for a foam mattress! Keep in mind any mattress you consider ideal sill be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions), and your PPP (Posture & alignment, Pressure relief, and Personal preferences). It seems like you have been doing some research, but if you have not already done so, you may want to refer to the
Mattress Specifications You Need To Know to the Mattress Durability Guidelinesto see how certain materials hold up over time.
For foams, Higher BMI sleepers need to be aware of the densities of the foam used if they choose memory foam mattresses, as any insufficiently dense memory foam can hide inexpensive ‘filler’ foam, which can breakdown prematurely, leaving ‘ridges’ and ‘valleys’ after a short time, which could exacerbate your back pain and can even cause alignment issues. We suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. This goes for all foam mattresses as well as hybrids; hybrids provide a different robust support system if it matches the sleepers preference in how it feels, but the
criteria for the top foam comfort layers is the same.
A few notes on the testing you have already done…most of the mattresses you describe fall into the ‘big brand’ category, in which specifications like density, materials used, and layer composition are obfuscated by marketing jargon and trademarked foams for which details are often nonexistent. Any mattress which does not disclose the density of their foams raises red flags for the informed consumer. You have also correctly determined the issue with in person restring - it can take weeks for any sleeper to ‘break in’ a mattress to see how it performs with their sleep profile, and yes a few minutes cannot give a proper sense of compatability! Best to focus on the specifications and therefore support and durability…this is one reason any good mattress manufacturer provides 90+ day sleep trials. It can also take a while for a sleepers body to adapt to a new mattress as well. And of course, the big brands have sales associates who will extoll the value of their particular mattress, as they commonly work on commission, and usually have no more insight into the products specifications than a consumer off the street…’built like a tank’ is certainly a strange way to promote a mattress; you’re not going into battle, you want to sleep comfortably!
Not sure where you are located and if you have access to any smaller independent mattress manufacturers…but if you are considering online purchases, you may want to consider the offerings of the Trusted Members of the site, many of who carry foam and foam hybrid mattresses in a variety of configurations, firmnesses and price points. These all have proven track records on quality, transparency in materials and construction, great customer guidance and generous return and exchange policies.
If you provide your Stats and any specific mattresses (that you have not discounted for various reasons!)that seem compatible, we may be able to comment on their suitability and durability.
Remember, a knowledgeable consumer is the best consumer…ignore the hype and consider the practical aspects as discussed above, and you will likely find your perfect sleep solution!
~ Basilio