Hi GeauxTigers.
Welcome to our Mattress Forum. 
Thank you for your kind words! It is worthwhile placing one’s energy in following the “road less traveled” that has integrity and truthfulness as its core values. I’ve seen the “demand” for such values growing slowly but surely. We spend a great deal of time educating consumers and providing the knowledge that will help them take out of the equation any guesswork and “dice rolling”. Unfortunately, too many consumers out there are vulnerable to advertising gimmicks, fake reviews, and more often than not an opaque and confusing mattress industry.
A DIY mattress can be both challenging and rewarding and it is best to be approached in a “spirit of adventure” where what you learn and the gratification that comes from the process itself is more important than any cost savings you may realize (which may or may not happen). You’re right that this way you can be sure of long-term durability and get very close to the perfect comfort/support for your needs and preferences.
Thanks for sharing detailed sleeping specifications and your mattress testing experience! This is obviously all very useful when it comes to gathering any meaningful data points to help guide your DIY build.
Indeed, latex seems to “fit the bill” quite nicely for most of your needs and preferences. A showroom latex testing will definitely confirm if you and your GF like the latex feel and eliminate one of the “unknowns”. I suggest you try both Talalay and Dunlop Of the same ILD on the same base. Generally, Talalay is very point elastic and “bouncy” and it’s probably the best material for avoiding the “stuck” feeling that you would get with memory foam, and has the ability to conform to the shape of a body, as you put it “with a small hint of sinking in the foam”. Latex is unusual for its ability to be both soft and supportive at the same time.
Kudos for doing in-depth research on TMU to learn the basics. You are again “right on” as stomach sleepers should choose the thinnest firmest comfort layers that are comfortable as sinking in too far can lead to a swayback position and cause back issues. Stomach sleeping is the most prone of all sleeping positions to sinking down too far in the pelvic area.
However, this directly “contradicts” the needs of side-sleeping, which requires that comfort layers generally need to be a little softer and/or thicker … to allow for your large shoulders to sink-in sufficiently to avoid any pressure point pains, tingling, or numbness. You would need to find the golden mean in terms of finding the right layer thickness and ILD for your comfort/support needs
You can read more about the needs of the various sleeping positions here, assuming you haven’t already encountered this in your research.
Great start for your DIY… I think it’s smart to consider a potential side by side split since there is a bit of a differential in you and your partner’s BMIs and she may require something a little softer than you. The good news is that this may not be necessary … with your GF’s petite frame, low BMI, and back/side sleeping positions would also need some good primary deep support and less comfort thickness. 2" of soft Talalay on top of 1-2" mid-layer and firm deep support should be sufficient for her to get the best of both worlds.
I wouldn’t go out of your way to seek a copper-infused layer - latex is in and of itself the most breathable foam on the market, and great for those who seek a temperature neutral material. I’ve seen some infused latex starting to crumble after some time of use. The additives in “Infused” latex are relatively new offerings and it is unclear how it affects the quality or durability over a longer period of time.
As far as “cooling purposes” go, layers that are closer to the top sleeping surface will have the greatest impact upon comfort and sleeping temperature (including your mattress pad, sheets, and linen). There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here.
Talalay latex has a high support factor of about 3 but Dunlop is even higher in the range of 4 so a combination between them would offer a range of flexibility and qualities that other foams cannot duplicate (most other foams are less than 3). Dunlop mid-layer in the same ILD would be more supportive. While many people would not notice a difference in feel between having a mid-Talalay layer as opposed to a mid-Dunlop layer, only you can tell for sure. Given everything you shared thus far, you should not need more than 4" of latex on top of the coils.
Because of your prone sleeping position and your GF’s petite 6" of foam may create some problems in terms of needed support which should be your primary concern. I’d start DIY building from the ground up. Generally, you can always add a softer topper if more softness is needed but you cannot fix a mattress that is too soft.
For your testing trip… It’s possible you’ve already taken a look at these resources, but you may find our article on Dunlop v Talalay in the support layers useful, as well as the counterpart about Dunlop and Talalay in the comfort layers.
We have a number of DIY experts on the forum as well who you may wish to reach out to with questions.
Ken Hightower at Arizona Premium is a great resource with a dedicated forum here on TMU and
Memory Foam Comfort is also a great option if you need to pick someone’s brain.
DIY Natural Bedding is a great resource too. While they are not yet an expert on TMU their level of expertise is exceptional. If you wish to reach them out through the forum let me know and we can drop them a line to come to your aid.
Of course, in-person testing will be pointing you in the best direction to go. Meanwhile, we’re happy to help you here in any way we can.
Phoenix