First, I don't necessarily agree with your date for Mark. "Generally agreed" is a fallacious term used to end argument.
But the point remains that Paul saw the existing scripture as sufficient to make the man of God complete, perfect, apt to every good work, what God intended him to be. Peter specifically refers to Paul's writing, and others, as that scripture. That more apostolic writings came after Paul's statement in 2 Timothy does not change the conclusion. There was a sufficient gospel before his death, and he assumed his readers knew what he was talking about.
What do you think he is saying? Are you allowed to suggest an alternate interpretation, or does that violate 2 Peter 3:16?
The date of John's writings have NEVER been in dispute.