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.
If I wanted to access my Jellyfin at home from a smart TV elsewhere, is that possible (securely)? Or would I need something that can run a vpn?