I know I'm in the minority, along with the OP. But I love the idea of Tatum, for this year's team, as a weapon off the bench.
I would start whomever Brad thinks is the best defensive 5 next to Grant Williams, specifically for his IQ starting the game...to get the team into the right flow...he doesn't need big minutes if not playing well or not effective...just be the kind of guy who does exactly what Brad is looking for to start the game at a high IQ level.
Kemba
Brown
Hayward
G. Williams
Best Defensive 5.
Then Smart and Tatum step in and either share or steal time from Brown and Hayward, depending on who's playing at the best, smartest level.
End of game you have 3 of Smart/Brown/Tatum/Hayward in the game....whoever had the least effective game on the bench at the end.... if two of those four guys sucked it up in a game you can go bigger at the end.
Next year Tatum back to starter (if not sooner due to how this season played out)....
edit: I not only don't like the small size to start the game, I don't like having so many offense first guys to start the game.