Confirmed. I could get the display off the board without damaging it and it was instantly clear that this cable isn't attached: https://imgur.com/a/iql0GXk
I put the cable back, and nothing changed. Write timeout.
Then I disconnected both cables - to try just the board alone with no screen. Write timeout.
Then I went as low as 9600, then 1200 bits per second. Write timeout.
So the board is brand new, it worked for a few days. My theory is that when that cable began to get loose, it badly one of the examples I uploaded. Now it's generating a ton of errors I can see in serial monitor. A permanent storm of errors. It renders the CPU so busy that it can't respond to serial, just pushes out the errors.
So I'd need a hard reset pin, that can wipe the entire memory of the board. Any hints would be appreciated.