Hi Sydney.
Welcome to our Mattress Forum! 
There are too many personal variables involved (not only your BMI, but also items such as your, somatotype, sleeping style, levels of flexibility, core strength, sensitivities, and preexisting conditions) to be able to diagnose a mattress issue or what could cause your pains. All I can speak of would be some general reasons for certain “symptoms” when sleeping upon a mattress (see post #2 here). Issues with “pain” earlier in the night tend to be surface comfort related, and issues with “pain” in the morning tend to be more alignment (deep support) related. While you don’t seem to express this as a cause of your issues, another common cause of back pain on a mattress can be something that is not supportive enough. Without knowing for sure if your pains are comfort or support related or the degree to which both are contributing, attempting to fix the mattress by adding a firmer layer won’t fix possible deep support issues that may have caused the initial back pain, but the good news is that you already got a few clues from your experiments.
In general when you add a firmer toper two main things will happen. First, the topper will compress under your weight and in combination with the layers below it will take the shape of your body profile. Firmer toppers will compress less, and create less of a cradle to re-distribute your weight and may result in pressure points. Second, the topper will “bend” into any softer foam below it and the “bending” will happen much more if the topper is firmer than the layers below. This is what I call a case of dominating layer, because the lower layers will compress more than the upper layer and the topper will “bend” into the dips, soft spots or the “compression” of the lower foam layers underneath, which will result in a sleeping surface slightly less conforming that has a firmer feel. This arrangement’s downside is that the recessed gaps are filled in a little bit less than if the topper was softer and the lighter parts of the body may not sink in as effectively for both upper body alignment and pressure relief. Support comes primarily from the deeper layers of a mattress and even though all layers work together and the thickness/thinness of the upper layers will also have some effect on your posture and alignment, this would not be nearly as much as the deep down support coming from the layers below.
This is also why it wouldn’t really be possible (except to a limited or temporary extent) to improve the deep support of a mattress where the upper layers are too thick and soft (or have softened or degraded) or the support layers are too soft by adding a firmer topper. The real solution would be removing or replacing some of the foam which has softened or broken down with thinner upper layers (not adding thickness with a topper) or replacing softer support layers with firmer ones (which can’t be changed with a topper). I uggest that before considering your new purchase or attempting to find another solution to “fix” the mattress you become confident enough that you’ve identified the issue(s) that cause the symptoms you are experiencing. IMO part of the challenge that you’ve been having with all of this is that as there are so many variables involved, as it’s often the case… that a “symptom” is only produced after a “cause” has been in existence for some period of time previously to the symptom itself. It will often take a while for a symptom to show itself. Sometimes it’s a matter of a “cause” that crosses a line in terms of degree or how long it has been there before the “symptoms” become clear.
While I or any experts on our site can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because 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 there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than 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).
Along with mattress shopping tutorial here (which includes all the basic information) 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).
Based upon the basic information you provided, that’s about as specific as I can be with my guidance. Hopefully, this brings you enough clarity to help with your next steps.
Phoenix