He has just posted a video of him receiving gifts from the enemy, and he is giving aid to the enemy. This is not speech, this is an attack on America's interests.
It's one thing to campaign in the US and say "I like it when Putin genocides Ukrainians", but it's another thing to be paid by a country that we're indirectly at war with, and provide publishing and broadcasting services to their president, a man who is on the US Sanction list. The illegal thing here is not the speech, it's the business transaction.
Subpoena Tucker's emails and phone and prosecute for illegal business transactions.