I think the easiest is just to set the S2C to factory defaults, configure API mode, set the baud rate, I use 57600, and the set it up in HA via integrations, it will auto configure from there.
Use xctu to configure the s2c, for confirming the usb path, I’m not familiar with that shield, but if it uses the pi serial, you are likely to have problems. The Bluetooth is connected to the same port and has to be disabled. The port would likely be ttyAMA0. I attempted to disable Bluetooth on my pi and use the onboard serial with little success and would recommend one of the USB adapters.
If you use Home Assistant and have access to a console or SSH, you can check out which device is being used
# discover serial device
ls -la /dev/serial*
# or
ls -la /sys/class/tty/ | grep serial
or
ls -la /sys/class/tty/*/device/driver
If you get permissions errors in the logs, it may be because Raspbian defaults to making a console available via GPIOs. You can disable this to allow your device integration full access to the port: