When you enabled the GPIO Serial port, you should also have disabled the console when it asked if you did it through raspi-config or the desktop configuration. (just checking you did), if you manually added it then you've missed a step, read Disable Linux serial console from the link already provided.

Since you do not intend to use bluetooth then add dtoverlay=disable-bt, to config.sys before all the serial stuff* and run sudo systemctl disable hciuart to stop the bluetooth service. This means your Serial Port is using a full UART, it will be available at the symbolic link /dev/serial0 and it now links to /dev/ttyAMA0 (normally it would link to /dev/ttyS0 the miniUART on a Pi4, read the bottom of that linked page).

*unfortunately this is still not an option in the serial setup in raspi-config so has to be done manually.