After not initially being able to see the printer using: ls /dev/serial/by-id/*, I simply disconnected and reconnected the USB cable and it came up. So it appears that the microcontroller was flashed. Physically opened up the printer and the MCU is a: GD32F303 with an 8MHz crystal besides it. According to this, the MCU should be supported with the options that I already provided when compiling but using Serial USART1.
So I reflashed again using these parameters, reconnected the USB cable, confirmed lsdevserial could see the device but no change to fault. I then reconfigured printer.cfg back to using /dev/ttyS0, restarted, reconnected USB cable, confirmed connection via lsdevserial but still no change to fault. I also tried it using the device serial number provided by ls /dev/serial/by-id but this also didn’t fix the fault.
The only thing I haven’t tried yet that I can think of is that I haven’t tried all of the other USART ports on the MCU. So I might try these one at a time sometime tomorrow, unless somebody can think of something I haven’t tried yet.