Despite freedom of speech, do you think that inciting a riot is a valid reason to suspend a Twitter account?

Because of freedom speech, no reason is needed to suspend an account.  Freedom of speech means that Twitter has the freedom to decide whom to allow to have an account and whom to ban.  Forcing them to allow Trump, or anyone, would violate Twitter's freedom of speech.

If Trump wants to speak, he can start his own website.  That is his freedom.  Not forcing his speech onto someone else's website.

Yes. This isn't "despite" anything. Freedom of speech guarantees a private business the right to decide what types of speech that they will allow on their platforms. To require otherwise is to compel speech. Government-compelled speech violates the First Amendment.

Well, they have deleted other accounts for inciting violence.