Hi AndyC79.
You are welcome! 
Your current DIY has a good probability of matching both you and your wife’s support/comfort needs. If I were you and could afford a little extra cost, I would do a side-by-side split with your wife’s side being a bit firmer. Aa she is a lighter individual and sleeps prone she needs a more supportive sleeping surface to avoid Hyperextension. Also, because she sleeps hot, a firmer side will prevent her from sinking in too much and trap the heat in a deeper cradle. A zippered cover is almost essential for a DIY in case exchanging layers or rearranging is necessary.
At your BMI you wouldn’t need more than 11" thickness, provided that you dial in as close as possible on the support/comfort levels needed for bor you and your wife. Even though I can’t tell for sure what would be appropriate layer firmness, with your sleep and pain history in mind, I’d suggest that you opt for a comfort level slightly on the firmer side. I’d keep in mind that you can always fine-tune a firmer mattress to get a softer feel with a thin topper, but it is almost impossible to fix a softer mattress to get the correct support needed for keeping a neutral spinal alignment.
That said there is nothing wrong with having a thicker mattress if this is high on your list of “must-haves”.
If you desire to increase your DIY’s height, I’d keep in mind that Thickness and Softness work together. This means that you may need to adjust the firmness levels of comfort and/or transition layers of your DIY… Because thicker layers (or mattresses) can have a greater range of compression and are more “adaptable” … it’s also possible to use firmer top layers in a thicker mattress and still have good pressure relief because of the greater range of compression of the thicker mattress which can create a mattress with a firmer “surface feel” but that still provides good pressure relief and adapts well to the body contours.
One other benefit of a thicker mattress that has multiple layers that can be rearranged or exchanged is that there are more layering combinations possible for changing and fine-tuning the performance and feel of the mattress but in many cases, this wouldn’t be necessary and in some cases can lead to a level of complexity that can make predicting how the layers interact more difficult (see post #2 here )
The overall thickness of a mattress (that is either “needed” or “preferred”) would depend on the combinations of the layers and components necessary to achieve the design goal of the mattress to provide the PPP (Pressure relief, Posture and alignment, and Personal preferences) that can best match each person and their unique body type, sleeping positions, and preferences.
I hope this adds more food for thought and helps your DIY research in search of a good night’s sleep.
Phoenix