I finally found a tetrahedrite vein, after exploring roughly double the area you can see above. In trying to express how much area I've covered, I realized you could describe the white-and-gray areas that are obvious extreme hills as "mountain ranges," in which case I'd only explored 3 before, and I found Cassiterite in range 5 and Tetrahedrite in range 6. Of course that understates how much time went into it, since each "range" covers a large map area when you're exploring it on foot.
It's been an interesting, if exhausting experience. The difference between this and vanilla reminds me of the difference between vanilla Factorio and Factorio with the Resource Spawner Overhaul. In vanilla minecraft, I never had to venture far from home. Dig down far enough and I'd find everything I could use. Here, I had to actively search for a considerable time, and it's going to be a long haul back and forth to my base. If I had the resources, and the vein is large, I'd be seriously tempted to build a railway to and from to simplify haulage use chest carts and to speed it up in general.
I'm not really set up to do that, though. The main issue is that making railway ties with Railcraft is extremely tedious by hand. For a track of this length (1700 meters from home, according the Xaero's Minimap waypoints), there's no point in doing it unless it was automatic. It's 1700 * 4 / 32 = 212 ties plus 240 steel rods.