A straightforward reading of the 1st Amendment indicates that there can be no such thing as illegal content under US law, including whatever you are right now considering proposing as an exception. An act involving speech may be illegal; if you make a credible threat of imminent, irreversible harm others are free to take you at your word and defend themselves—the justification here is the harm which is reasonably expected to follow, not the content of your speech. If you lie to someone to obtain their property under false pretenses, knowing that the lie precludes any "meeting of the minds" and thus renders the contract invalid such that the property still belongs to them, then you are committing theft. Your punishment derives from the act of taking property which did not belong to you, not the fact that you lied in the process. And of course what you say may be used as evidence against you without the speech itself being illegal.
There are some more problematic areas where the Constitution itself is inconsistent. Copyright should not exist, for one; the core concept is utterly incompatible with freedom of speech. The Supreme Court even recognized this at one point—it's why we have the concept of "fair use" in the first place—but "fair use" is a poor compromise which does not fully negate the infringement of the freedom of speech. When you have one clause saying that Congress has the power (but not the obligation) to do something which would infringe on the freedom of speech, and another clause later passed as an amendment saying "Congress shall make no law... abridging the freedom of speech", the obvious reconciliation of these clauses is that Congress is barred from exercising that enumerated power because it would violate the later amendment. The Court tried to strike a balance instead… but it still amounts of Congress passing a law which abridges the freedom of speech, despite the limitations imposed by the Court.
There was one other notable exception to free speech that undermines your point: obscenity is not, or has not been, considered speech under the terms of the free speech amendment. So, at least historically, there is some precedent for considering that some forms of human expression can be censored on their own merits.
I should note that I am anti-censorship and consider such laws absurd, but my point is that we can't rely on readings of the constitution to self-evidently protect us from such things.
Many poor precedents are grounded in strong emotional reactions at odds with basic principles, and in my opinion obscenity cases are a good example of that. Still, the classification as obscenity is more about the form of the speech than its content. In fact the more content there is, in the form of either expression or artistic value, and the more the form contributes to accurately conveying the content of the speech, the more likely it is that the speech is considered protected, even if the form would otherwise be considered obscene. I'd rather the courts didn't get involved in trying to decide whether a controversial turn of phrase has enough merit to warrant protection, but in any case I don't think you can extend the principles underlying prohibition of obscenity to exercising control over which information can be conveyed.
obscenity is not, or has not been, considered speech under the terms of the free speech amendment.
What part of "shall make no law" isn't clear? It's true that obscenity is an exception to 1A, but that's something that some people made up after the fact, in direct contravention to the framers' stated intention.
The founders weren't short on ink. If they had wanted to equivocate, they were certainly free to do so. The fun really starts when some people decide that their (least) favorite amendments are subject to less (more) interpretation than others.
There are some more problematic areas where the Constitution itself is inconsistent. Copyright should not exist, for one; the core concept is utterly incompatible with freedom of speech. The Supreme Court even recognized this at one point—it's why we have the concept of "fair use" in the first place—but "fair use" is a poor compromise which does not fully negate the infringement of the freedom of speech. When you have one clause saying that Congress has the power (but not the obligation) to do something which would infringe on the freedom of speech, and another clause later passed as an amendment saying "Congress shall make no law... abridging the freedom of speech", the obvious reconciliation of these clauses is that Congress is barred from exercising that enumerated power because it would violate the later amendment. The Court tried to strike a balance instead… but it still amounts of Congress passing a law which abridges the freedom of speech, despite the limitations imposed by the Court.