After a lot of testing of different leathers, splits, papers, inks, nibs, etc. I am convinced that I most prefer the full-grain cowhide, unbacked.
I'm using a sample of tanned cowhide from a full hide I used on another project. It's an ideal combination of cushion and support for my kind of writing. I sampled inks from PR, Noodler's, Diamine, and Waterman; I found no variations in their common characteristics on a notebook or pad.
The papers I used were Rhodia, Clairfontaine, Strathmore Writing 25% and 100% cottons wove, handmade Colonial Williamsburg letter, and a couple sheets of random stuff. The only paper, ink, nib combination that was unpleasant was a Post-it note and my Aurora Ipsilon F with Diamine Deep Magenta. Turned out though, that I just hate post-it notes.

The leathers I sampled were full-grain buckskin, top-grain buckskin, and various splits of cowhide. Over and over, the unanimous winner was the full-grain cowhide. It's firm but gives a slight cushion, the suede back stays still on my desktop, and the deep oil-tanned top-grain held the paper neatly but not stubbornly. No oil or pigment pulled up into the back of any sheet I tested.
To go the distance, I also tested two rollerballs, one fine and one medium and found both to work deliciously on the full-grain cowhide. No poke-throughs, skips, or snags. I deliberately attempted to poke through and it worked with only one paper— if you guessed Post-it note, you picked the right one.

Conclusion: I know what I want as my mat surface, so the next step will be to work with Art on dimensions and aesthetic appeal. The question also remains about what steps I'll have to take to keep it flat. I am not opposed to clamping it between a couple boards once in awhile.
The objective of my pursuit is to find my ideal, desktop single-sheet surface. This smaller piece will be set aside when not in use. I do not like writing on tablets/pads or thick things that interfere with my level hand/wrist support. This is just a thing with me.
I'll report back with the next installment when there is one.
One caveat: Yes, when one retires very early with a disability, these projects become paramount. LOL!