Mr. Gary~ I realize I am late to the party here, but would like to add, if I may.Reading through this entire thread, there are a couple of things that came to mind. They seem to have been addressed, but will reiterate, for what it is worth.
When I first started with leather in the late 70's, there was not much to choose from. Certainly no way to try and find options in the dinky town I was in. So through the years have tried countless threads. I use Ritza at this point in making. Like the strength, wax, etc. as do many others. However, I never really cared for the flat design of it. I have a time trying to keep it from twisting. So thought a 'rounder' type thread would be nice, if possible. (what do I know about making thread?) Reading the review from Mr. Richard, it appears you may have addressed this.
As for the size,, I usually use .8 or 1.0, but would like the option for .6 on small items. Maybe, if it all works, down the road you could convince them to add a bit smaller one to the lineup. For the next step in your plan, the sewing machine thread, I do use #69, but only for fabric linings when requested. Actual leather is #90 or #138. I hand stitch the thick stuff. And as mentioned, the machine requires a left twist thread. I think most industrial machines have a lube pot, or other lube system, mine does not, don't know if or how that would figure into the thread making.
I don't use a stitching groove, so the thread sits on top of the leather. After hammering down, it doesn't sit proud, but it isn't in the protected groove either. So a thread that can take a bit of abrasion perhaps. I imagine somewhere along the line, that is one of the tests you will run.
The number of colors you are considering is pretty impressive.
Sounds like you are coming along nicely with your project. Look forward to giving it a try. Wish you the best of luck.