Having done this lots with Mrs ACS
1) we rarely to never check bags
2) we get the wheelchair from the airport. DO NOT wait in the wheelchair area for someone to be the pusher… you’ll be there for a long time. They have chairs there But they insist they are “reserved” and a staff is coming momentarily to use them for someone waiting. It’s rubbish. Just tell them you can’t wait you’re late and take one
3) if you are able be the pusher. The rest of the staff at security and US customs are mostly very helpful and kind
4) they will “check” the chair to make sure it’s safe to take airside
5) once airside you have 2 options. Continue to be a pusher or use the motorized system. We prefer to have me as the pusher. Way faster and more flex especially for lounges …
once you give up a chair they aren’t easy to get. Not impossible but not easy….

in the USA if you’ve prebooked wheelchairs they are usually waiting at the gate and as the US is tip based economy $10-$20 gets you great service. The staff push it and will wait to get luggage if you have or get you to wherever in the airport you are going and fast

coming back same in US
once you’re landed in Toronto there are always in our experience chairs at the gate and you can either get in one at the gate and stay in it, or use the motorized transport at the top of the ramp to get to customs where there are dozens of chairs and you take over again if your can
The motorized system is a bit slow. Usually requires some waiting but it’s a long push from plane to customs hall most of the time

hope that helps