Hi TalkingDog,
Welcome to the Mattress Forum! 
While I can certainly help with “how” to choose, it’s not possible for me to make specific suggestions or recommendations for either a mattress or combinations of materials or components, as there are just too many unknowns, variables, and personal preferences involved that are unique to each person. The first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress, and you have some very specific needs for your mattress. The most reliable manner to make a decision is based upon your own careful testing (hopefully 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), and you do have a few experiences that you listed and based upon that I’ll try to offer up a few ideas that you may find assistive…
[quote]My first bed was from SleepEZ. They were as helpful as they could be, but no combination of layers quite satisfied me
Firm (bottom) - Medium - Soft let my butt sink in too much, although I liked the feel apart from that.
Firm - Medium - Medium, even with a 1.5" memory foam topper (Aerus) the bed had too little give
Firm - Soft - Medium, my butt was sinking in, with or without the topper[/quote]
You didn’t list if the latex was Talalay or Dunlop, or if you tried a mixture of both. I’m assuming these were the 3" layers. And I’m guessing the topper was the 1.5" Aerus memory foam topper. After reading through everything you stated a dominant layer on top seems to make sense (at least with this configuration) and you stated that was what you liked the best. With everything you stated, and again not knowing the mix of the latex, I would have been interested to learn what a firm, medium Talalay, medium Dunlop, and 2" plush Dunlop topper would have felt to you. But that bed is gone, so what was learned from what you tested was that you did like some plushness on top, but with your dimensions and preferences you need a supportive transition layering, but not too hard.
[quote]Next I tried a Saatva, which I still have.I think I prefer the spring-feel over the latex-feel
I got Saatva’s Luxury Firm. It’s borderline OK still (at two months), but I expect it will just get worse. I now need to move to the center of the bed, where I haven’t yet been compressing the padding, in order to have that feeling.
I think part of what I like with the Saatva comes from their doing something to strengthen the lumbar area.[/quote]
You certainly may have an affinity for the feel of steel versus foam for the support core of a mattress – there really is no one correct answer, as long as the componentry is of a high enough quality. Polyfoam will take more of a set than latex and there will be more of a “false firmness” associated with that type of product. The Saatva does have a middle third zoning with the base Bonnell spring unit, and there is also some zoning in the foam layers with a bit of memory foam and zoning in the quilt. This certainly can provide a bit more durability in the area you desire, but as you’ve discovered the polyfoams will soften (they do use about 2" of foams that are lower quality than I would normally like to see), and you’re already switching to the areas of the mattress where the foams haven’t already lost their false firmness, so this may not end up being right for you as well over time. I guess you’ll have to wait and see about that.
Based upon your previous comments and current experience, there may be a few things you’d want to investigate. One I talked about previously and you tried, and it’s something I term using a dominant layer in the uppermost layer of a mattress. This is using a bit of a higher ILD foam above a lower ILD foam for a bit more of a “supportive and taut” surface comfort. There is more about dominating layers if you click here.
A second thing you may wish to consider, and you’ve also brought this up, is a zoned mattress, allowing for the slight sinking in that you desire but something that still can provide support and comfort. Various zoning systems can also be very useful and worth considering for people who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoned mattress construction here and zoning in post #11 here. There’s also a detailed thread about reverse zoning here that you might find interesting.
Flobeds, a member her of the site (which means that I think highly of them) offers some very unique zoned systems. Berkeley Ergonomics has some mattresses that use innersprings with latex on top that are also zoned and configurable. As zoning can get quite technical and complicated, if you are considering pursuing a system like this your next best step would be a detailed phone conversation with any vendor that does offer a zoned system and to use their guidance in assessing your needs.
Another thing you’d want to consider, as you use your mattress as a chair as well as a mattress, would be to make sure that anything you chose used good quality componentry in the upper layers, specifically latex foam, polyfoam over 2.0 lb. density, or micro- or nano-coil spring units in the upholstery layers.
While I can’t speak to whether any of these suggestions would be a good “match” for you in terms of comfort, firmness, and PPP, they all use good quality materials and there are no lower quality materials or weak links in any of them that would be a reason for concern in terms of the quality or durability of the materials.
I hope that information helps you out a bit. I’ll be interested in what you decide to do with your current mattress and if you try any other options.
Phoenix