Personally, I would aim for future sight on your Zam. Let it be the psychic glass cannon it's meant to be. 😁
The only reason focus blast and shadow ball are viable on Mewtwo is because Mewtwo's only STAB charge move is psychic, which is a fairly underwhelming charge move. If Mewtwo was able to learn future sight, then the other two options would become borderline obsolete.