Again, I realize that the amendments are worded differently. What I am claiming is that there's a reasonable argument that perhaps the principle behind the First Amendment should include the prevention of such contracts.

Also, I will note that you point out that the specific words are important in your first point, but miss it your second. You mention that the First Amendment is about not allowing the government to regulate speech. But the actual text says that Congress is not allowed to "abridge" freedom of speech. Abridge means to limit, shorten, or cut off. Clearly, a law that invalidated contractual limits on speech would not in any way be abridging that speech.