Hi worknman,
I’m sorry to hear about your rib sensitivity. Unfortunately, because each person is different in terms of their physiology, weight distribution, and body shape as well as the positions they sleep in, their individual preferences, sensitivities, levels of fitness and flexibility, and the specifics of any issues they may experience, there is no way for me to be able to determine why you would be feeling the way that you are on any particular mattress, or to be able to recommend a specific combination that would suit you. If I could, I’d provide you that information in a heartbeat! 
There is some information about the many different symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful. You seem to mention that a softer surface comfort works better for you. While a softer surface can provide initial relief, you’d want to be sure that it doesn’t allow you to sink in too deeply (I think previously adding a memory foam topper to your BME mattress caused a problem), as this can lead to a higher concentration of pressure to your thoracic (rib) area through the night (and impact support), especially with memory foam that tends to get softer as it gets warmer. Conversely, a mattress that is too hard on the surface can result in equally uncomfortable pressure points in your rib area.
Some people with difficulty finding comfort in particular areas of their body have chosen zoned sleeping systems with positive results, but this can be complicated in configuring and I’d recommend the assistance of a qualified retailer or manufacturer if you choose that route. There is more about zoning in this article and in post #11 here. Also in post #2 here and post #7 here (latex monozone unique properties). Latex is very point elastic, and you have stated in the past that you liked it for an upper comfort layer, and I wonder if an all-latex system might provide you better comfort as opposed to a mattress using a polyfoam support core.
If you’re experience such discomfort after lying down for only a few minutes, to me this tends to speak a bit toward your personal sensitivities/flexibility, as there should be some products that can provide you relief for longer than just a few minutes.
Phoenix