Hi Denver Shoulder Pain,
Normally a bunkie board wouldn’t help much on a firm non flexing foundation unless the foundation under the mattress was sagging but some people can be much more sensitive to a very small amount of sagging so it’s good to see that this helped you.
My “guess” would be that this is probably an issue with the firmness of your support core and if I was in your shoes I would still lean towards a firmer support core and then build the additional softness and pressure relief you need on top of it.
Unfortunately … the only way to know whether this would be successful would be based on your actual sleeping experience because you are somewhat “outside the averages” where “theory” wouldn’t be an effective way to predict what may work best for you.
Having said that … I would probably try some of the suggestions in post #11 here first which would be a less costly way to experiment and would give you some idea if zoning will help solve your issues before starting to cut up layers.
The better sources I’m aware of for mattress components, layers, and “pieces” are listed in post #4 here.
Phoenix