Okay, I _might_ have fixed things on my Mac with the following:
1) another Mac
2) a good USB-C cable
3) an app called "Apple Configurator 2" (available from Apple)
4) an activity called "Reviving the T2 chip" on the bricked Mac
I'm both amazed that such a recovery activity exists for a bricked device (and is available to the end-user), and wondering how I even _needed_ it.
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