Hi davess,
While it’s possible that your current platform could be part of the issue if it is sagging under your mattress … I don’t think you will need a bed rug on top of your Charles P. Rogers Medera platform which should be fine by itself.
I would also be cautious about assuming that your pain is connected to the mattress if your symptoms just appeared “suddenly” because it could be connected to some type of movement or twisting during the course of the night.
While I can certainly help with “how” to choose … as you mentioned I don’t 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” or PPP (Posture and alignment, Pressure relief, and your 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 accurate 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).
Having said that … your approach certainly makes sense to me.
You’ve probably seen this but just in case you haven’t … the better options or possibilities I’m aware of in and around the San Francisco/Berkeley/Oakland areas (subject to making sure that any mattress you are considering meets the quality/value guidelines here) are listed in post #2 here.
Phoenix