Update from Nov 4 at YYZ:

arrived at 525am for an 8am departure at T1.

there is a holding area where they call flights to enter security (beginning around 6am). It’s based on departure time.

Nexus is open. We were able to go directly there and line up outside the entrance (ropes are setup). A little after 6am they open and we got through security.

Customs opens closer to 630/645 so there was another set of lines there.

All in all, flying first thing in the morning to US sucks right now!