I'd persuade the wife to run the dishwasher before leaving. Kill the
power to the HWT when she starts it.

Yes, I even installed a switch on the water heater for that purpose

When the dishwasher cycle ends, turn off the water feed to the HWT.

I'm not fond of killing power to the pump - you might lose prime.


Prime has never been a problem; it always pumps up in a minute or two.

There are filter on it now, but the water heater is apparently still
full of
old debris that no amount of flushing seems to eliminate completely.

Drain HWT. Remove the lower heating element. Tape a short length of
garden hose (or whatever will just fit into the heating element port)
on the end of a shopvac hose, insert into HWT and suck it out.

I will give that a try, thanks