Just a general observation:
Of all the mechanisms that a team might use to improve, trades are the hardest way.
Trades work the best at correcting a team imbalance. Someone as good as Jaylen Brown is never that type of problem.
Smart, Fournier, Langford and Nesmith are an imbalance. That's 4 of our top 8 players who play shooting guard, and that's Jaylen's best slot too.
We need two complimentary players, some normal player development and we are one of the top 5 teams in the league. If we just had a pretty good power forward, we would improve dramatically.
I generally agree with you with the exception that Smart's best position is PG. However, Kemba's best position at this point is closer to the 2 than 1, so the imbalance persists.
If you believe that about Smart then you either peg him as a starter or a backup point guard. I don't see how he can be a starting point on a championship team unless we had one or two elite passers at other spots that you can run an offense through.
Pairing him with Walker has been a disaster in my opinion. The team pushes responsibility over to Tatum, who is a great scorer but hasn't been a good enough passer to elevate his less talented teammates.
I'm OK with him serving as backup point, but that means he's a utility backup, and would need some of his minutes off point, or he's playing only 15 minutes.
Without getting into potential trades or cap implications I would be interested to see how Smart would do at PG if the Cs added a big wing with some passing ability and retained Fournier as the 6th man.
Example
Start Smart, Brown,Tatum, Nance, and R Will
Bench Fournier, Thompson, Pritchard and Nesmith/Langford
With this sort of roster I would look for continued improvement creating from Tatum and Brown. Particularly for each other and getting away from the your turn my turn basketball. With an offseason and new contract I would like to see Fournier pushes into more of a PG role. His career best is 3.6ast per game with a change in role (2nd option to bench role and 3rd option) I can see him pushing that above 4 per game.