SOLVED
The serial settings are fine, I just had the wrong display type defined in the firmware. I'm a very paranoid person apparently.
Thanks everyone.
Original post:
I have acted recklessly and now I need help.
I tried to update my Ender 3 Pro with an SKR V1.3 to Marlin 2.1 and I tried to skip some steps.
I couldn't find a v1.3 example in the configurations repo so I used a v1.4 instead because it had the right processor.
I changed some stuff over but didn't give it a thorough look and I didn't change the SERIAL_PORT 1 and 2 settings.
When I tried to load the new firmware from the SD card the screen was blank and it hasn't worked since.
The firmware file changed to a .elf so it maybe installed?
I wonder if I can even load via the SD if I've super messed up it's serial config. I tried some other firmware versions plus BigTreeTech's from 3 years ago but I don't think they're being read.
If I've messed up those serial configs I wonder how I could undo it without the SD card. Help...
P.s. does anyone have a copy of Marlin 2.1 that works with the SKR V1.3 or the config files for that board on the Ender 3 Pro?
The SD card reader has nothing to do with the serial port, so you're fine on that point.
I would just try and roll back to the bigtreetech firmware for the 1.3, even if it's for an older Marlin. Once thats working, build Marlin 2.1, updating the configuration to match the known working one.