You definitely wouldn’t be following the formal flow as you are not expected to enter Canada when you connect to the US. Canada may take issue with you entering without your bags, but is possible you could just tell them. If you short checked your bag you could immediately go upstairs and check in and then leave the airport before entering the US. It would be a little tight, but you would have time to even head into to the city on The UP Express or visit a restaurant Near the airport. Best opinion is to go carryon to YYZ and then check it for your last leg.