After quite a bit of testing, I have found out a few things.

1) there is a burn in period, where the contacts need to be worn in, before the system comes to stable values, and

2) there is a lifetime before the values start to drift out from the previously stable values.

This is expected, but it seems that for the materials I chose, the time to failure is too short, about 10,000 cycles. The burn in is maybe too long, 150 cycles or so. In between, the thing works very well! If I could widen the range to 0 to 10 cycles on wear in, to 100,000 cycles on wear out, I'd be happy. So it's an order of magnitude on either end. That seems doable. Perhaps not easy, but certainly not impossible. A material change could get an order of magnitude or more. I am investigating.