... I am also pretty certain that our canonical Mark was not the first gospel narrative.
I don't recall your reasons for it, but I agree with the above statement in the sense that the first person to mention Mark (Papias) says that Mark wrote down "whatsoever he remembered of the things said or done by Christ" that he heard from Peter.
EH 3.39.15:
Mark, having become the interpreter of Peter, wrote down accurately, though not in order, whatsoever he remembered of the things said or done by Christ. For he neither heard the Lord nor followed him, but afterward, as I said, he followed Peter, who adapted his teaching to the needs of his hearers, but with no intention of giving a connected account of the Lord's discourses ...
I think you might mean that Mark used written sources though, and I have no opinion about that other than that I suppose someone could have written down what Peter (or any other apostle) said about "the things said or done by Christ" and Mark could have used it as a source in addition to "whatsoever he remembered" that Peter said.