Upon re-reading the post I noticed you're in Toronto.
We might have some of the same road conditions. You do indeed need to worry way more about the ice and compact snow. Coworkers with RWD vehicles got stuck a few times in the icy parking lot divets, and the Model 3 would be no different.
That said, I did pizza delivery in winter in my smart fortwo (also RWD, fairly ideal weight distribution too). It was fine with winter tires and more or less routinely plowed roads. I couldn't get up some icy driveways that the Honda Fit could with ease, though. Traction control only gets you so far (and turning it off only gets you so far as well).
If you see other "low" vehicles getting around just fine, chances are the Model 3 will also be fine. Just don't decide to become a snowplow with the heavier snow.