Interesting turn. Actually several of them.
Last night, last gasp, did a factory reset of the phone, then restored. Left cellular data on overnight and while it didn't gulp data, it still used over 2mb in a few hours. Since it's been so variable, I don't think that counts as a fix.
Then I stumbled onto Apple's "Shortcuts", which I had previously dismissed as a useless gimmick. I was wrong. I created a location based automation shortcut that turns on cellular data when leave home, and a second one that turns it off when I arrive home. Nice! Problem (mostly) fixed!
Then I got an email from Red Pocket's eBay store. They surprisingly refunded me 11 months of my 12 month purchase. Go Red Pocket!
Also Red Pocket expects to be able to offer eSIMs for Verizon towers...at some point in the future. I'm considering switching to Visible on their Month to Month plan for a while (since it's $15/month for a year) and see if Red Pocket actually does get Verizon eSIMs. Since my current Red Pocket plan now ends mid February, I have a little time to ponder.
As mad as I've been at, well, everyone (Red Pocket, Apple, AT&T), this is a better outcome than I expected.