Thanks for additional inputs and insights. A few additional notes and thoughts on some of the questions:
= Disc vs caliper; while it isn't on top of my bias list, the reason behind my bias is that I am both heavy and cautious. As a result, I use my brakes a lot. As a result, after many kilometers of cycling, I eventually wear down the rims. Photo below was my last Russia trip where the rim had split after many kilometers. Someone lighter or less cautious might make other choices.
= Number of speeds in cassettes. I was using 8 speeds on my Trek mountain bike because that is what it came with (bought in 2011). I was using 9 speeds on my Trek 520 because that is what it came with (bought in 2007). At the largest cities in Russia in 2009, 9 speeds wasn't an issue. In Latin America in 2016, I found 8 speeds but also got impression 9 was also relatively available. So no strong biases, and in part my idea with starting with recent mountain bike is to use whatever it comes with as stock.
= One doesn't need three touring bikes. Absolutely agree.
= Frame stiffness. I've use two Aluminum bikes on extended tours. One was the mountain bike I showed at start. The other was a Cannondale 1000 that I cycled around Australia in 2001. In both cases the tubes were a bit larger than steel might have been and I believe this helped rigidity/stiffness. All else being equal, when I've loaded things down, have liked the ride on both. My Trek 520 (steel) flexes just slightly more but it isn't a big deal. However, given that the mountain bike frame eventually cracked was reason for my (weak last on the list) bias of steel over Aluminum.