Thank you, Luke! That's exactly the information I was looking for. The winner is ...
- ubuntu-mate-desktop
To confirm, the sequence to use would be as follows:
[1] Install "Ubuntu mini ISO" ... from
[2] Install minimal MATE-related
sudo apt-get install tasksel
sudo tasksel install ubuntu-mate-desktop
Is that correct?
My "Grand Plan" ...
My intent is to take some time to dive in and understand the various "ISO builder" tools out there and, once I have my environment finalized, create a set of scripts that would allow me to create a "logical snapshot" of the pieces that I have customized, and have that apply an overlay of an on-disk ISO image, so that an install will rebuild that customized environment with whichever version of Ubuntu comes along.
I am planning to review each of the following, to see what "code pearls" I could harvest for re-targetting to my purposes:
- Distroshare
- Ubuntu Imager
- UCK
- Cubic
- Respin
- Remastersys
Given that intended purpose, I don't know why a robust tool, which performs that very function (applying a customized overlay onto a Distro install ISO), doesn't already exist.
I had previously attempted to use the "Ubuntu Imager", but that was a flop. It was so long ago, I can't remember why.
This time around, I have the time on my hands, so I plan to pursue that to its proper completion. I just don't know how long it will take me.
(I hesitate to pursue the Nix/Hydra approach without having an actual working reference model, to see what is done and how, for comparison. Even though everything that has been shared by Martin thru the Ubuntu Discourse seems very promising, I honestly don't have the sense of confidence in my ability to learn that "ecosystem" from scratch. I always try to work from abstractions, wherever possible, but I fear the language used for the description of those abstractions for Nix may be too abstract for me, even though I haven't dug into that so far. I will give that a quick look ... during the next few days before I finalize my decision to avoid that approach ... until after I constructed a first tool using my intended BASH approach. I am trying to avoid a full Distro build from source code libraries, in order to maintain the flexibility that referencing a pre-existing Distro image would provide. 
)
Would you know if "Yocto" is more along the lines of a source-based system? Or is it more a "middleware" option like what "Nix" seems to offer?
PHASE I - Structured download of BUILD-related files