I flew CHC-CAN-SYX-LHR last month.
Can thoroughly recommend avoiding SYX routes.
The CAN-SYX leg is domestic, so you have to go through immigration and customs at CAN, then back through security to a couple of special gates in a separate part of T2 for SYX flights. The gates are downstairs, small, cold, with toilets and no other facilities/shops. My tip is to exit to the check-in area after immigration & customs, rather than following the transfer signs. There are plenty of shops and cafes here, and it's a 15 minute walk through security to the SYX gates. If you have lounge access, you can only use the first class checkin lounge (rather than the excellent international transit lounge), in departures zone C opposite the domestic security gates. This is just a small waiting room with a coffee machine: no toilets, no showers, no hot food. The staff did go through security to bring us hot food from the domestic lounge. Lounge staff walk you to the gates at the last minute, which is nice.
At SAN, international transit passengers are given badges, put on a separate bus to the terminal, and escorted through security and immigration checks and left at the departure gate. Business class passengers got a lounge pass but economy elites got nothing.