I agree with you that marijuana is harmful. But the federal government never had the constitutional authority to outlaw intrastate marijuana. That's why I menitoned the 18th Amendment in conjuction with intoxicating beverages. The states had delegated to the feds the power to regulate intrastate alcohol only between the times of the ratification the 18th and 21st amendments by the states.
The reason that it was bad for the feds to unconstitutionally regulate intrastate marijuana by prohibiting its production is that the states likewise needed to grant Congress the specific power to do so via the Constitution, as opposed to the feds wrongly dealing with marijuana outside the framework of the Constitution as they have been doing.
Also, FDR's activist justices wrongly gave the green light to Congress to overstep its limited Commerce Clause powers and interfere with intrastate commerce. This is evidenced by the fact that the Supreme Court had previously clarified that intrastate commerce is off-limitts to Congress.
”State inspection laws, health laws, and laws for regulating the internal commerce of a State, and those which respect turnpike roads, ferries, &c. are not within the power granted to Congress. (emphases added)” —Gibbons v. Ogden, 1824.
Otherwise, I think that the feds reasonably have the constitutional authority to define marijuana as a narcotic for interstate Commerce Clause purposes.
Prohibitionists, of course, will completely avoid the constitutional issues raised in your post.