As alluded to in previous posts, you do not need to go via YYZ on AC - you can get a Star Alliance award (for the same Aeroplan miles as a pure AC award) on a combination of UA/US/AC - the US airlines tend to more likely have confirmed business class seats, compared to AC.

Another strategy, if you want to go only on AC; you might want to try to go to Fort Lauderdale or West Palm Peach - easily drivable to Miami. Half the time when I stay in Miami, I fly to/from FLL rather than MIA.

Another option: you should not limit yourself to fly via YYZ; you might try to transit in YUL. I do not know if AC flies (it used to fly once a week between YOW and FLL in the winter) to southern FL via other hubs - you may check on that.