Well, after 2 days of banging my head against a wall, going through maybe a dozen different what-to-do-if-you-get-100%-disk-activity help pages, all of which gave more or less the same checklist of things to turn off or twiddle with... I finally started systematically shutting down every program I have that were launched at startup... stuff like all the instances of Skype, Discord, Telegram... the lot... until I got to the Pi Node app and to Docker Desktop (which links back to my Pi Node app, and of which I have been running for months) and... soon as I quit out of Docker Desktop, the 100% disk activity quickly dropped away to 0%.
What in the petunias of heck?!?!??
(((Hurries off to the Docker Desktop help forum on Reddit))