I got a new USB cable and the error has changed. I’m using the firmware that @Arketype prepared as we seem to have the same board. This is extremely confusing because the firmware is definitely loaded and the pi definitely detects the printer board but for whatever reason they refuse to talk to each other.
Tail of the klipper log: Build file /home/pi/klipper/klippy/../.config(3105): Wed Jan 25 12:24:58 2023= - Pastebin.com
Edit:
I got a brand-new monoprice USB cable today and tried that. It still didn’t work, but I at least got more descriptive errors: mcu 'mcu': Starting serial connectmcu 'mcu': Unable to open serial port: [Errn - Pastebin.com
Edit 2:
The new error was because the serial device ID/path changed. I changed it to the new value and it went back to mostly silent timeouts. I think I will try to reinstall mainsail on the pi, I’m running out of reasons to blame it on the printer/firmware.
Edit 3:
Reinstalling mainsail did not work. Still see the device, but no connection.