Trump and his cult members are claiming he can say and think whatever he wants about the election — it was stolen, rigged, a do-over is needed, etc. That’s not what the case is about — and the indictment recogniz(s)es Trump’s right to say and think those things — but about what he he told/asked/ordered others to do. That’s the “conspiracy”.
Now he wants to argue that asking others to commit crimes is protected speech as well. And the Supreme Court already ruled in 2008 that it isn’t. Again, it’s the asking that’s the conspiracy. You can’t ask someone to commit a crime — like, say, election fraud. That’s illegal. And not protected speech under the First Amendment. So saying it is free speech won’t work as a defense either.
And I doubt very much even this Supreme Court would overturn that 2008 decision (US vs Williams) if it came to that (which it may, knowing Trump). It was decided 7-2 with the opinion written by the most conservative justice on the bench, Antonin Scalia.