This will have virtually zero effect. The president only has the power to pardon people convicted or accused of violating federal law and prosecuted in federal court. The problem is that nearly all people arrested for simple possession of marijuana aren't charged under federal law nor prosecuted in federal court. They are charged under state law that similarly outlaws marijuana possession and prosecuted in state cour…
Our constitution does not define the breadth and depth of emergency powers and it would be up for both congress and SCOTUS to decide that the president can’t do that.
I would hope that a president would just say fuck it and try this though. As much as I want things to be done “the right way”, I value lives of folks more than my neurodivergent desire to have things done neatly.