I agree the Cs are best served adding a knock down shooter to their bench rotation. Ideally a 4/5 type bigger stretch 4 who can play spot minutes at the 5.
One player who I might be worth looking at is Markief Morris, he is currently making 3.2mill this season with an option for 3.3mill next year that he will likely decline. With Detriot looking to likely blow it up he could be an interesting fit. Maybe Poirier and a late 1st for Morris? (assuming the parting with Marcus Morris is viewed as amicable in the locker room).
Its also worth considering that the playoff rotation will likely feature 9 players.
I see this team as having 6 locks to play every playoff game (if healthy)
1 Kemba Walker
2 Jaylen Brown
3 Gordon Hayward
4 Jayson Tatum
5 Daniel Theis
6 Smart
After that..
Kanter, would be a lock except he can be forced of the court based on match-ups
Wanamaker, has been super solid but doesn't really move the needle (could be vital if any injuries to top 6 - theis)
G Williams, shoot will dictate playoff viability
Semi, gets minutes vs the Bucks
R Williams, wild card coming back healthy and producing could see him force his way in as the 7th guy
Looking at this roster with the playoffs in mind its clear how players like Morris, Bertans, or Bjelica could be very important for a deep playoff run