Hi Axiom81,
Welcome to the Mattress Forum! 
I’m glad you’re finding the information here on the site helpful and informative, and I hope it assists you in making an educated decision.
I’m sorry to hear that you’re having some lower back issues. The DreamFoam Ultimate Dreams 13" Gel mattress uses good quality materials, but it is definitely on the more plush side of the spectrum, and it is possible you may do better with something using more resilient upper comfort materials to assist with your lower back pain.
As you mentioned, there are too many individual variances involved for me to be able to predict what might work best for you and your particular situation, but sleep ergonomic research would tend to support your supposition that something with either a bit firmer deep support, or more resilient upper comfort layers, or a bit less in the upper comfort layers – all options focusing upon obtaining as neutral an alignment as possible – could be a good direction to pursue.
You can read more about the Loom & Leaf in the Simplified Choice Thread here. It uses 2" of convoluted 4 lb gel memory foam over 2.5" of 5 lb memory foam, on top of a transition layer of 2" polyfoam that rests upon a 5.5" polyfoam support core. As Saatva has told me that these base polyfoam layers are 1.5 – 1.8 lb, I would treat them as 1.5 lb. This model would be more closely aligned with your current DreamFoam mattress in componentry.
Their mattress is unique and they are constantly updating their algorithm for mattress layering for their clients (changing the order and firmness of their comfort layers). They use information gathered from some of the most respected PHDs and sleep ergonomic researchers in the industry, so I do think highly of how they attempt to go about customizing their mattress the achieve what they feel will be the best possible results for their customers. The componentry they use would be suitable for your BMI. You can also read more about the Helix in the Simplified Choice thread here.
I’m not familiar with or in touch with the ownership/management of Nectar, so I can’t comment upon anything but their componentry, and I would still advise caution until all of the details about their mattresses (as I’ve commented upon previously) are known.
The build of the Alexander Signature is again similar to your current DreamFoam mattress. In the soft, it uses a 4" 1.8 lb polyfoam core, above which is a 3" 1.8 lb polyfoam layer, on top of which is a 2" 4 lb memory foam layer, a 2" 4lb gel memory foam layer and 1.5" of polyfoam quilted to the covering. Being a softer item and closer to what you currently have, I don’t know if this is necessarily the direction you might which to pursue. The Alexander Signature Hybrid would use a pocketed spring support unit, which will have a different feel and support characteristics than a polyfoam core. On top of this is a 2" 1.8 lb polyfoam layer, with 2" of 4 lb copper infused memory foam for the Luxury Firm (3.5 lb for the Medium), and topped off with 1.5" of polyfoam in the quilt.
Regarding cooling memory foam claims, you can read more about phase change materials in post #9 here and at the end of post #4 here) and you can read more about the various different types of gel foams in post #2 here. In general terms gel foams will tend to have a temporary effect on temperature while you are first going to sleep until temperatures equalize but have less effect on temperature regulation throughout the course of the night.
Copper itself is a very thermal conductive material so it would make sense that copper fiber infused memory foam (or other types of more advanced thermally conductive materials such as graphite) would be a little more effective than the gel that is more commonly used but it would also depend on the amount of copper that was in the memory foam.
In very general terms … gel and/or other thermal conductive or phase change materials can have “some effect” on the sleeping temperature of a particular material but how much of an effect they will have and how long the effect will last will depend on the specific formulation of the material and on the “combined effect” of all the other materials and components of the sleeping system including your sheets, mattress protector, and bedding. Many thermal conductive or phase change materials tend to have a more temporary effect when you first go to sleep at night or over the first part of the night than they will over the entire course of the night.
It’s not really possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material.
Phoenix