I agree that making your own is easy. A good portion of mine are made from bolts. Stainless bolts are certainly the best to make them from but you can get by with other materials too.
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.