Hi Kristenebean04,
Welcome to the Mattress Forum! :).
If you haven’t read it yet … the first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you with how to make the best possible choices … and perhaps more importantly know how and why to avoid the worst ones.
As you mentioned both a mattress that is too soft and a mattress that is too firm can cause support and alignment issues and like the three bears the goal is always to find a mattress that is not too soft and not too firm but is “just right” in terms of both support/alignment and comfort/pressure relief. Too much or too little of either one can mean a mattress that isn’t a suitable choice in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
I would tend to avoid “theory” though because it can take a long time to become an expert in mattress design and theory and the learning curve never really ends. The most effective approach is normally to use careful and objective testing (using the testing guidelines in the tutorial post) where your body can tell you which mattress is a good match for you along with the guidance of a knowledgeable and experienced retailer or manufacturer who already knows what you would otherwise need to learn.
Unfortunately I can’t feel what you feel or see you on a mattress and there are too many unknowns, variables, and preferences involved for anyone to use “theory at a distance” or specs (either yours or a mattress) to know which mattress may be best for you (see mattress firmness/comfort levels in post #2 here). Having said that … outside of the links there if you are interested in some of the theory behind mattress design then post #4 here has more information about primary and secondary support/alignment and their relationship to comfort/pressure relief that may be interesting to you. There is also more in post #6 here that can help you visualize what good alignment “looks like”.
The quality/durability of the materials in a mattress will also play the most important role in how long you will sleep well on a mattress and post #4 here and the posts it links to has more about the many variables that can affect the durability and useful life of a mattress relative to each person but it’s also important to make sure that a mattress you choose isn’t “on the edge” of being too soft for you when it’s new because then only a very small amount of foam softening can take you “over the edge” and even a mattress that uses relatively good quality materials may not last you very long (see post #2 here).
The better options and possibilities I’m aware of in the Oklahoma City (including Jim at Oklahoma Mattress) are listed in post #2 here.
Phoenix