Further update…
I was able to update the firmware (both .otz and .bin) on the last of my dimmers, all went smoothly. The otz file took only about 10 minutes for this dimmer, and the bin file took only about 2 minutes. (This seemed to be about the same speed as the first dimmer.) I suspect that something about the 2nd dimmer, or perhaps its location, was causing the firmware update to proceed much more slowly than for the other two. Strange, but at least it’s working now.
Thanks, folks, for your help!