Unless you root your phone, you can't disable the stock keyboard. Even if you root and you decide to do so, be forewarned: you might lose access to your phone. If, for any reason, the keyboard of your choosing fails to load at boot, with no stock keyboard for the system to revert to, there's no way to access your phone if it's password-protected; even if it's fingerprint protected, at boot you always have to input your password with the keyboard. Only option, should that happen, is to connect a physical keyboard via OTG.