You may want to contact Lonnie Height, who is running the show now at Gomph-Hackbarth. I talked to him about a year ago and he was still selling tools. 520-642-3891 or on his Facebook page https://www.facebook.com/people/Gomph-Hackbarth-Tools/100021212726476/ . He might be able to answer your question.
I use a 1' x 1' piece of marble for a lot of my tooling. It has worked well so far but it is developing a fine crack across the middle. I also have a large slab of slate I use for larger pieces. It's pretty solid but I don't dare hit really hard on it. Your idea of quartz is a good one but like you mention, it's thinner. I have been tempted to get two slabs of quartz bonded together to make a thicker piece. I'll probably pursue that one of these days.
I use oil dyes, mostly Fiebings or Lincoln. I add alcohol to get the shade I want. Before I apply the dye, either by dipping or wiping (heavy coats when wiping), I wet the leather with alcohol first. I never get blotchy finishes doing it this way.
Barge doesn't work the way it used to. None of the formulas I've tried compare to the original formulation. I have changed to Master Cement and it works like the original Barge one did.
I have the same groover you showed. It's easy to use and does the job well. My only objection is that the smallest groove is bigger than I sometimes want so I have to resort to an old one I have on hand that I don't like as much but does the job.
I made my first stitching awl by grinding down a screwdriver shaft. I keep it around because it reminds me of how I started out. I now get Vergez-Blanchard awls when I need a new one.
This is some of the best advice you can get.
2:45-3:34 shows a great use for an X-acto knife. I use his method more than any other for cutting complex shapes from bridle leather.
A good holster making video.