My original project has been operating for a while in a satisfactory way. I do still have one unsolved problem (knowing when the pump stops pumping), though for a while I've had some ideas about how to tackle that. In the meanwhile, some clever and industrious people over in the Home Assistant forums have been making progress on reverse engineering the wire protocol between Navien's NaviLink unit and the tankless water heater. I think that will give me the answer about the pump (not sure). To find out, I'll need an RS485 interface to the water heater control board.

If you look around for an RS485 interface board (as some of the folks in that forum thread have done), you come across some components from M5stack. Their stuff is particularly appealing for one-off projects since it mostly comes in reasonably attractive packages. I already have a little box full of assorted M5stack stuff, so I decided to make use of that and a few things I had to order from them to re-implement the hardware.