2FA Using A USB Key

The easiest way to achieve what you describe without knowing if you need to unlock the screen or decrypt your disk is to program a USB key (Yubikey) to automatically type your unlock password when the button is pressed. This way you can have long secure random passwords for both your LUKS encrypted disk and your screen lock password. This approach will work without changing any software in your computer because Yubikey can simulate a USB keyboard to type. However this is not very secure because anyone with access to your key can copy the password by connecting it to a computer. The right way to do it requires changing software to implement a cryptographic protocol between the host and the key. New operating systems will support FIDO U2F, Webauthn or similar standards to achieve what you describe in a real secure way. But you can use the automatically type password now if you get a Yubikey and you use the personalization tool available in Ubuntu Software.