Kemba, Brown, Tatum, Hayward, Theis, Smart
These guys will get most of the minutes
Wanamaker, I expect will get regular time but not much.
Then pick among the following for spot minutes depending on matchups:
Semi, GRANT, Kanter, Timelord (if healthy)
Nobody else should see the floor unless it's a blowout.
In particular I would say that Kanter should probably not see the floor against a quality opponent unless it's the Sixers. Anybody else will likely be able to pick and roll him right off the floor.
The ideal bench for this team would swap out Wanamaker, Semi, and Kanter for a VanVleet / Barea type guard, a PJ Tucker type wing, and a solid two-way big a la Amir Johnson circa 2015.
There's no realistic way for the Celts to revamp the bench like that, though. Even so, I think they can go far in the playoffs, depending on matchups.