@droftarts Thank you again for your help!
While I can't run a test print right now, I think I may have figured it out, thanks to your earlier hypotheses. And if so, it's a doozy...
I think that the issue is that the extruder wire loom was too close to the X axis stepper motor, and the motor may have been causing EM interference that propagated back to the board, resetting it during certain moves...
I tried manually moving the X axis back and forth while the loom was distant from the motor and I didn't observe a single reset. But if I set it back to where it was (right up against the housing), sure enough, I was easily able to get a reset after a bit.
This would explain why the resets only ever happened during moves, and why they started all of a sudden — I must have moved the electronics box slightly, and with it the loom.
I'll be sure to follow up tomorrow, when I get the chance to run a full test print with the loom away from the X stepper!
I'd never been able to print that long without a failure, so I'm hopeful that's the end of that!