Hi drderek.
Welcome to our Mattress Forum! 
As a back sleeper, you are correct that you need a firmer and more supportive sleeping surface. Back sleeping has a slightly less curvy profile that is not quite as deep as in side sleeping and the “gap” that needs to be filled in with the comfort layer for pressure relief is generally the small of the back (the lumbar area) which is why a slightly thinner and firmer top layer will generally work a much better. In other words for back sleeping a slightly shallower cradle is needed. A good starting point for a back sleeper is 2" of softer material on top of your mattress and then increasing or decreasing from there depending on other factors like weight, preferences, or other sleeping positions.
I am not sure what’s in the comfort layer of the Posture Premium IV but regardless of the brand generally mattresses that were made that long ago tended to use higher quality materials (in the comfort layers especially) than the mainstream mattresses made today (see post #3 here ). Even though you do not perceive any “sinkage” it’s almost sure to have lost some of comfort layers’ height and firmness since you purchased it even if the foams don’t show any visible impressions. (this can be confirmed by testing ILDs of the materials in your mattress for firmness/softness). If you were to open up the mattress you’d probably also discover that the foam (or batting) inside had some minimal compression but not enough to make any meaningful difference in comfort or support. If your mattress was two-sided (which were also the norm at that time) then if you flipped the mattress on occasion it would also even out any impressions and allow the side that wasn’t in use to recover and even though it would still be gradually softening over time, it would soften more evenly over the entire surface so it wouldn’t be as visible.
While there may be some slight impressions (that you may or may not notice over time) a firm hybrid latex mattress that is a suitable match for you in terms of PPP will last you for many years. You can see a video here of a latex mattress that was in use for almost 50 years (although most people wouldn’t keep any mattress for nearly that long because their own needs and preferences would also change over time and 50 years for any mattress isn’t a realistic expectation) and both blended Talalay and 100% natural Dunlop have a long history of lasting decades (see post #9 here ).
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research 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 perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve 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”, firmness, and PPP (Posture and alignment, Pressure relief, and your own 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 (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
The criteria you previously listed fits a wide range of products so unfortunately, it is not possible to predict and point you to a specific mattress or combination of materials and components you will do best with or that you will sleep best on with any certainty based on specs or “theory at a distance” …I would be very skeptical of anyone who claims that they do but you seem to be heading in the right direction in planning to do some local testing… you’ve been through quite a few mattresses in the last year and you’ve collected enough data points that will be helpful in your discussion with trusted manufacturer/retailer to find a suitable and long lasting mattress. Some good local testing will also give you a much better sense of the many different types of materials and components that are used in mattresses and some reference points about the types of mattresses (see this article) and confirm the firmness level you tend to prefer which can help you narrow down your choices regardless of whether you end up purchasing locally or online.
At a quick glance some of the Trusted members of our site that carry products that fit your description are Flexus Comfort and Nest Bedding. Flexus has their manufacturing facility in Covina, and Nest in Los Angeles and also CA on your I10 or 210 commutes that you can visit and whose products would meet the criteria that you are looking for in a mattress and will go to great lengths to ensure that the mattress is suitable for you. Also going the other direction on I15 you can visit the Mattress Makers that has some double sided firm options such as the Kensington firm that you may wish to test. I’d certainly do some local testing and rely on the guidance of a knowledgeable manufacturer or retailer that can help you narrow down your choices.
If you are interested specifically in latex or latex hybrid mattresses then this link in the tutorial is to a list of the members here that sell mattresses online and many of them sell latex and latex hybrid mattresses that use different types and blends of latex that have a wide range of different designs, options, features, return and exchange policies, and prices that may be worth considering. Post #3 here also includes a list of many of the online manufacturers that sell component latex mattresses.
Phoenix