Don't think leather comes smooth on both sides, one side is smooth the other is like suede. Sometimes the suede side is pressed smooth, but it is not smooth enough for stropping, in my very limited experience. I have only made 2 strops to date.

Horse butt cordovan would be nice, but TM looked into it and it was not economic.

I would go with latigo, bridle leather, or a butt leather with a satin (ie not polished finish. This is what i think the guys in the know suggest (ie Tony Miller and Keith De grau). Alternatively get some samples from your leather chum and suck it and see.

May i ask what do you think massive thickness will achieve?