Single best thing you can do for that trip is to splash out the $100 each way for "preferred seats" (usually bulkhead or exit-row) on the YYZ-HKG legs.

Think of it as about $9/hr for a considerably more comfortable flight.

You can definitely apply the $200 Amex Plat Travel credit towards this.

If you haven't booked yet, also keep an eye on the fact that Premium Economy is only about $150 more than Economy, per leg, if you connect through YVR.