So being a "witness" is restricted solely to your in-person verbal testimony? If so, that seems unreasonable.

Nope. They can't make you write out a confession. They can't make you record a confession. They can't compel you to become a witness against yourself.

If you've already made documents that incriminate you, though, without being forced to by the government....well, all that is fair game. Absent some other protection or evidentiary shield, of course. If they find your diary or get a wiretap and record you confessing to your crimes to someone, they're allowed to use that stuff - even though it is your words they're using. And of course, if you choose to tell the government information, then that is also fully admissible.

The protection under the Fifth Amendment is against being compelled to testify against yourself, not against using stuff you voluntarily and willingly created or said in a criminal case.