Hi thinkTwice,
There is more about the different ways to choose a mattress or layering combination (either locally or online) that is the most suitable “match” for your specific needs and preferences and how to identify and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them in post #2 here.
It sounds like you did a good job of local testing to find a layering combination that has a high chance of working well for you. 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 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 would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing or personal experience … hopefully using the testing guidelines in the tutorial post (see mattress firmness/comfort levels in post #2 here).
Beyond a certain point then any further combinations that you tested would likely have begun to blur together anyway so I would trust the testing you did and use your actual sleeping experience to decide whether you need to make any further changes to your default layering.
With the layers you have you would have a number of options available to you to change the comfort or support of your mattress by rearranging the layers if you need to without needing to exchange a layer. For example it’s very unlikely that your wife will feel much if any difference if you replace her bottom medium layer with one of your firm layers so you would have the chance to try (from top to bottom) medium/soft/firm/xfirm which would give you a little firmer sleeping surface or you could even borrow a second medium layer from her side (and replace it with your soft for long enough for you to test it) and try medium/medium/firm/xfirm which would give you a firmer sleeping surface yet. There are also other combinations that would be available to either one or both of you as well so at this point I wouldn’t second guess your choices and I would use your actual sleeping experience to decide whether you need to rearrange any layers and how to rearrange them and your experience with different layering combinations would also give you the experience you would need from actually sleeping on the mattress to decide on whether you need any layer exchanges as well.
For now … the best thing you could do would be to actually sleep on the mattress with the combination of layers that you chose 
Phoenix