The justification is Trump wants to do to Canada much what Putin is doing to Ukraine. Russia being allowed to take Ukraine legitimizes world powers engaging in conquest within their sphere. It's really that simple.
The talk about fentanyl, the supposed trade imbalance, and all the rest is just smoke and noise, because if he said that he wanted to raid Canada for resources and Lebensraum it might actually threaten his support.
He's not being subtle in this. He and his people have made constant comments that Canada doesn't work as a nation, that the border is artificial, and even explicitly that the tariffs would just go away if Canada became the 51st state. This is economic warfare driven by manifest destiny.