Hi mt_huckleberry,
As I stated in my previous reply, there are unfortunately far too many variables involved for me to predict what mattress model or brand may best suit your needs. I can only assist you with learning how to evaluate a product, but after that nothing will replace your own personal testing. There are too many unknowns, variables, and personal preferences involved in choosing a mattress for someone else to make specific suggestions based on specs (either yours or a mattress) or theory at a distance that can possible be more accurate than your own careful and objective testing (using the testing guidelines in the tutorial post) which is always the most reliable way to predict which mattress will be the best match for you in terms of PPP (see mattress firmness/comfort levels in post #2 here).
I or some of the more knowledgeable members of the site can help you to narrow down your options, help you focus on better quality/value choices that are available to you either locally or online, help you identify any lower quality materials or weak links in a mattress, act as a fact check, answer many of the specific questions you may have along the way that don’t involve what you will “feel” on a mattress, and help with “how” to choose but only you can decide which specific mattress, manufacturer, or combination of materials is “best for you” regardless of the name of the manufacturer on the label or whether anyone else (including me) would have the same criteria or circumstances or would make the same choice.
Unfortunately, there would be no way for me to know if any of these mattresses would be “too firm” for your shoulders – no one can predict your own personal preference via an online forum.
My Green Mattress ( a site member here, which means that I think highly of them), has quite a few mattress offerings, so being of a “slighter” build, you may wish to phone them and ask for their suggestion for one of their items that allows for better surface contouring to allow your shoulder to “sink in” a bit more while still providing good deep support. (I see in your follow-up post that you did exactly that – excellent!)
Tuft and Needle uses two different layers of polyfoam (you can see their specifications in the Simplified Choice thread here), and the density of the foams would be suitable for someone of your BMI, but I wouldn’t know if you would enjoy the comfort.
The specifications of the Purple mattress are also listed in the simplified choice thread. It also features componentry that shouldn’t be an issue quality-wise for your BMI, but like the Tuft and Needle it comes in only one comfort choice.
OSO (also a site member here, and their specifications are listed in the simplified choice thread as well) does have the option, as you mentioned, to have a softer area under the shoulder zone and the density of the foams would be adequate for your BMI.
Phoenix