Hi audiohead,
There are many companies that offer a 6" latex mattress (or a 6" core) both locally and online. The Tutorial post here includes a link to some of the members here that make a range of latex mattresses including several that are 6" thick or have a 6" core that provide the option of using different types of latex or different configurations that you can choose based on your budget or on your firmness/softness needs and preferences (although it appears that you may have read this and found most of them already).
A 6" latex mattress (or any thickness) will certainly be durable because latex is generally the most durable foam category (memory foam, polyfoam, or latex foam).
In terms of firmness this would depend on whether you are talking about the surface firmness, pressure relief firmness, support firmness or overall “feel” of firmness (see post #15 here about the different types of firmness). The type of latex, the thickness of each layer along with the thickness of the mattress itself, and the type and ILD of the latex along with your own body type, sleeping positions, and preferences and sensitivities will all play a role in how firm a mattress feels for you. You can read a little more in post #14 here about how thicker layers or a thicker mattress can affect the feel and performance of a mattress.
In general the two ways to choose the design of a mattress that is best for you is either your own careful and objective personal testing on local mattresses or a more detailed conversation on the phone with online manufacturers that includes as much information as possible about you and your local testing so that they can help you choose a mattress that has the best chance of success based on “averages”.
This would depend entirely on your own personal value equation and on all the objective, subjective, and intangible factors that are most important to you. I certainly wouldn’t use “consensus” as a way to choose a mattress because only you can know the specifics of what is best for you and consensus opinions don’t take individual criteria or preferences into account. I would talk in more detail with each manufacturer you are seriously considering (they will know more about their mattresses than any consensus opinion) and spend “enough” to make sure your mattress purchase includes the materials, features, and options (before, with, and after a purchase) that you prefer and can comfortably afford not so much that you are including features or options that are less important to you when you are making your final choice between “good and good”.
You certainly have many options listed in your list (it reminds me of the research I did as well when I had way to many “final choices”) but the good news is that none of them include any obvious weak links in the mattress although there are many different designs and all your choices aren’t apples to apples comparisons in terms of materials, design, features, and options. I would use your local testing for PPP on different latex mattresses as reference points to help you narrow them down.
Phoenix