Yes hard reset changed nothing.
I've talked more with local Cox techs and after considerable discussion they've scheduled a truck roll after my work time on Monday. Our pedestal is in the neighbor's yard and I've let them know Cox may be doing measurements in their yard then.
I'm not crazy about a service call, but I also can't justify tossing out a random $200 on a new cable modem, especially if there really is a legit signal issue that would explain the sudden speed drop.
FWIW, all the cabling in the attic was done by Cox years ago, so no homebrew connections or jury-rigging.
