Hi gibo,
I would keep in mind that a mattress isn’t designed to support the more concentrated weight of sitting … only the more evenly distributed weight of lying down. This is especially true of more “point elastic” materials such as latex or memory foam. If you spend time sitting or “propped up” on your mattress then it may be an idea to consider an adjustable bed or a wedge that can support some of your body weight under your back.
I’m not sure what type of support system you have under your mattress but if it has any flex then it can make a difference in how your mattress feels and performs. You can get a good sense of what a suitable support system would feel like by either putting your mattress on the floor for a few days or by putting a sheet of plywood under your mattress (if only temporarily … see post #10 here) to see whether there are any improvements.
diynaturalbedding’s suggestion would be well worth trying IMO as well because back stiffness can come from muscles that are working too hard over the course of the night and this can be the result of comfort layers that are either too soft or too firm and sleeping on a firmer sleeping surface will help you clarify the direction you may need to go in terms of firmer or softer.
If I’m reading your feedback correctly it seems that the M/F/S combination was an improvement compared to the F/M/S combination which may be “pointing to” changes in the direction of firmer but you would need to confirm this based on your own experience.
Phoenix