Northern Light
Superstar
Yonge has Beecroft and Doris. Maybe Sheppard needs its own service road(s) between Yonge and Leslie.
Sheppard isn't really the problem here; its Bayview.
There are 2 straight-forward answers here; but they aren't cheap; and even less popular.
Bayview has a relief road already, in theory, Willowdale Avenue.
Its long enough, wide enough, and rarely has heavy traffic.
But the issue is traffic going to/from the 401, particularly from the north.
To divert the some of the cars to Willowdale; you need a 401 interchange w/Willowdale Ave. A full interchange (but going/coming from the north only) would be ideal; but even just an exit-only interchange would be a help.
At the same time, to build transit market-share, you need to build Willowdale Station. That was part of the original design for the Sheppard line; and its absence makes transit less attractive in the area.
But both a new highway interchange and a new subway stop would be opposed by a majority in the immediate vicinity of Willowdale + Sheppard, as well as those living right on Willowdale. It would take a very brave politician to point out the solution and then stick their neck out to get it implemented.