If erosion can be an accomplice, then I agree with your idea here. Regarding humans, had Brad told anyone, it seems Val would have been top of his list. But my guess is that he told no one, since he seemed to be doing all the critical steps alone and, often, with his phone disabled. I think stats for these types of crimes also suggest that he likely acted alone.
BTW, I've been looking how far Brad could have travelled away from the highways that it seemed he used to go back and forth from Boerne to Medina. Obviously, when his speed on the highways and with the disposal are high, then he has more time to travel away from one of these highways. Although he could go either north or south (away from the mostly-east-and-west highways), I think it's more likely that he would travel north to go to a disposal site, because his ranch was north of TX-16 and because most of the spots that look good for this near his ranch are north of TX-16.
All I want share in this post is that routes that go north a few miles from TX-16 (say to a more distant part of the Medina River, or even to small nearby lakes, which would have been mostly dry at this time) can't be ruled these out. BTW, these lake beds appear to be rocky in a similar way to the riverbeds, although the terrain of the lakes are smoother, so they don't offer as many hiding spots. Again, erosion could have been his accomplice, where all he needed to do was pick out a low spot, deep enough for the blue tarp (with the large bulky item still inside it), cover the tarp with rocks, prep the concrete at/near the truck, carry it over and pour it, hide the site by using more rocks or other debris, and take away any "spent tools", where the only "spent tool" not already back in the truck was possibly just the orange 5-gallon Home Depot bucket, now with concrete residue on it.