This is why I use a jailbrake tweak that let's me draw a passcode after initial unlock, but requires a full alphanumeric password on reboot or after 5 ties. I have no clue why Apple has yet to incorporate that feature directly in the OS. Not sure if the fact that the phone being jailbroken is a further vulnerability, but to be honest, I don't really worry about FBI, as much as I am concerned about loosing my phone and having someone find it and open it.

The feature is there. Change the 4-digit pin in the Preferences to an alphanumeric password, and the iPhone will work exactly as you described.

There's no built-in feature that lets you use a simple passcode after the initial unlock.

Touch ID ends up working like this, where your fingerprint takes the place of that simple passcode. But for people who don't want to use a fingerprint, you're stuck using the same passcode or passphrase for each unlock.

There is no pattern password in iOS. It's only a numerical or long form password.