Why do you need to turn it off? Just use whatever keyboard you want. The default one can not be removed for good reason, I assume it's to prevent accidental lockout after restart as the GrapheneOS devs can not control whether or not your third party keyboard has an issue which prevents it working when your phone is in BFU state. You could get locked out of your phone completely, especially with the USB security controls it's possible to put in place.
So just leave the default keyboard on in the background and install and use whichever keyboard you want
And to switch between keyboards there is usually a little round button in the bottom right of the phone while the keyboard is active