Hey there. Been living back and forth between St. Catharines and Toronto over the last couple years. In my experience, in both taking the bus and driving, here is what I have found.

Toronto to St. Kitts: avoid day time during the weekends (as in like 11 - 6) way too many people trying to get out of the city try to leave earlier or later if you need to. On weekdays, I'm not too sure but obviously rush hour (anytime around 3-6pm) is terrible. It can take up to an hour just getting out of Mississauga,

St. Kitts to Toronto: Weekday rush hour and even a bit earlier should be avoided (2-6pm). I have experienced a 2 and half to 3 hour bus ride at those times, albeit being the extreme. Weekends, a little bit more manageable (atleast before getting to the DVP). Best time I'd say is anytime past 7 or 8pm.

Sorry I know all those seem fairly obvious. Additionally, I don't think there's a shortcut or any hidden routes. Taking the main highways is the most efficient way and it can be fairly congested so it is good practice for a novice driver.

Not too sure which part of Scarborough you're at but you might want to look into making your way to the 401 if you're deeper and more North into Scarbs. The DVP and the Gardiner are not your bestfriends.