Hi k2166 and welcome to the Mattress Underground 
I’m so sorry to hear of the health issues and pain you have been experiencing for some time…you have very specific needs in a supportive, comfortable mattress and you are correct, if your mattress is not causing your pain, it’s likely contributing to it.
Let me start with the basics: any mattress you find comfortable and supportive will be based on your Stats (height, BMI, sleeping position(s) and any underlying health concerns) and your PPP (posture & alignment, Pressure relief, and Personal preferences); It might benefit you to take a look at the Mattress Specifications You Need To Know compared to the Mattress Durability Guidelines to see how certain materials will last over time.
As for your specific situation, a few thoughts. First, your experience with some of the ‘big brand’ mattresses are unfortunately not uncommon…these manufacturers deal in volume, and there can be very little ‘customization possible for consumers…they typically lack transparency in the composition, density, and even thickness of their mattresses, making it almost impossible to assess the suitability or durability for any given sleeper.
From your description of your mattress history, you have experienced both too soft comfort layer(s) - which for a higher BMI sleeper can allow you to sink through the top layer and ‘bottom out’ on the firmer layer(s) below…as well as too firm - where the mattress does not adequately allow for proper support on your pressure points.
As far as mattress materials, for foams, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
The big brand mattresses you have had experience with were possibly of insufficient density for your body profile, causing the memory foam to prematurely break down causing ‘ridges’ and ‘valleys’ in the foam which can definitely cause muscle and joint aches and back pains and even alignment issues. Your past discectomy is another variable in the equation.
Latex may be a solution; though not everyone enjoys the feel, it’s more breathable and much more durable than memory and polyfoams, and can allow you a lot more customization of the firmness of the comfort and support layers.
Your description of your mattress history is excellent as it does provide a lot of ‘data points’…the toppers, the camp cot, etc., for a good manufacturer, tells a lot about your needs. For example, a mattress with ‘zoned’ comfort and support may help as you need varying levels of support for your specific sleep profile.
As I am by no means a medical professional, I would not want to guess where your pain caused by physical issues ends and mattress-caused pain begins; not sure what price point you are thinking of, but you may want to take a look at the offerings of the Trusted Members of the site; these have all been vetted for quality, transparency in construction and components, a great deal of experience in assisting in finding consumers their ideal mattress, as well as great customer service and generous return and exchange policies. They carry are a variety of different mattress types at different price points, and their reputation rides on every sale, rather than a huge marketing budget driving sales. If you see any in your price range, contact the to see what they suggest, or of course if you find any that you find appealing, you can feel free to ask us here and we can perhaps comment on how suitable and durable they may be for your specific sleep profile.
I certainly have empathy for your ordeal, it sounds very frustrating and painful. I hope this helps you, and hope for success in correcting the much less than ideal sleep solutions you have been living with for quite a while. Please keep us posted how your search is progressing.
~ Basilio