Hi halfwhitts,
It’s probably a bit of both. Stomach sleeping is the most risky of all sleeping positions because the pelvis tends to sink in too far into the mattress relative to the rest of your body which leads to sleeping with a hyperextended lower spine and the discomfort and back pain that can go with that. This is even more risky with mattresses where the upper layers of the mattress are too thick and/or soft.
The foam softening under the pelvis would also make things worse because your pelvis would be sinking down even more. Age can also play a role because as we get older our spines are generally less flexible and not as tolerant to “out of alignment” conditions as they were when we were younger. The increased wisdom of being older has its tradeoffs 
With side sleeping pressure points are more of an issue so comfort layers generally need to be a little thicker and softer.
It’s also not uncommon that sleeping positions change over time and if you are comfortable without sleeping on your stomach (at least as a primary sleeping position) it would probably be beneficial for your back.
I’m guessing you’ve already read this but just in case … post #1 here has all the basic information, steps,and guidelines that can help you make the best choices.
There are far too many variables and unknowns for anyone (including me) to recommend a specific mattress design for someone out of all the thousands that are available based on “theory at a distance” that could possibly be more accurate than your own testing using the guidelines in the previous link. My goal is to help with “how” to choose rather than “what” to choose. There are links to some of the theory involved in mattress firmness/comfort levels in post #2 here that can help give you some insights into some of the ideas behind different designs and what to look for but these are only generic and not meant as specific suggestions for any particular person or body type because each person is different.
I’m also guessing you’ve seen this but for others who may be reading this the better options I’m aware of in the Portland area are listed in post #2 here although Parklane has a wide range of mattresses that are among the best quality/value available and I would strongly consider them.
Phoenix