Here's the issue:
How many minutes do you plan on playing the starters of Fultz, Bradley, and Hayward? Because if you plan to play them 30 minutes each (which is on the low side for starters generally), then you are giving IT, Smart, and Crowder all only 18 minutes per game... and you haven't even budgeted any for Jaylen Brown yet.
The issue goes far beyond starters/bench. Minutes available is the problem.