Hi ritatown,
The first place I would begin is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you with “how” to make the best possible choices … and know how and why to avoid the worst ones.
I would keep in mind that the specific design of a mattress and how well it matches your own unique needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is much more important than the type of material you use. There isn’t one material that is “better for bad backs” than another in other words because it depends more on the design of the mattress.
Your circumstances and health issues would likely make finding a new mattress more complex and challenging than the norm so before you consider designing and building your own mattress I would read option 3 in post #15 here and the posts it links to so that you have realistic expectations of success and the steep learning curve, trial and error, and costs that may be involved before you begin. If you have a “spirit of adventure” and the the DIY process itself and what you learn along the way is more important than any cost savings (which you may or may not realize) and you have no sense of urgency and are unlikely to become frustrated with some of the unsuccessful designs that you initially put together as you progress towards your ideal design then it may be worthwhile but it certainly isn’t the simplest or easiest approach to buying a mattress.
You may have already seen this but just in case you haven’t the better options and possibilities I’m aware of in the Fresno area are listed in post #2 here (including A Better Bed). There is also a longer list for the Monterey/Salinas/San Jose/Santa Cruz areas in post #2 here but these would involve a longer drive.
I’m not sure why your mattress was different from what you ordered but if the only issue is that a mattress is too firm then it may be worthwhile adding a topper based on the suggestions in post #2 here and the topper guidelines it links to.
I would also keep in mind that only you can feel what you feel on a mattress and when you are making a local purchase then your own careful and objective testing using the testing guidelines in the tutorial post will be the most reliable way to know whether a mattress is suitable for you. There are too many unknowns, variables, and personal preferences involved for anyone to be able to predict which mattress you would do best with based on specs (either yours or a mattress) or on “theory at a distance” (see mattress firmness/comfort levels in post #2 here).
There are also some suggestions in post #3 here and the posts it links to that may be helpful for for larger body types or higher weight ranges.
If none of your local options are attractive to you and if you are open to an online purchase (which I would assume you are since you were already considering a DIY mattress that you can’t test in person ahead of time) … then the tutorial post includes several links to the better online options I’m aware of that would also be well worth considering and talking to on the phone.
Phoenix