Trade I think makes sense where it doesn't cost us the TPE (and isn't my go to favorite trade involving Collins and Barnes)
1st trade
Boston - Robinson (into Poirer TPE)
Sacramento - 2021 2nd Rounder
2nd trade
Boston - James, Jeffries (into Kanter TPE)
Sacramento - 2022 2nd Rounder, 2023 2nd Rounder
3rd trade
Boston - Barnes, Whiteside
Sacramento - Theis, Nesmith, L. Williams, Kabengale, Patterson, BOS 21 1st, BOS Cash
Los Angeles - Thompson, Teague
So final summary
Boston In - Barnes, Robinson, Whiteside, James, Jeffries
Boston Out - Thompson, Teague, Nesmith, Theis, 2021 1st, 2021 2nd, 2022 2nd, 2023 2nd, Cash
Sacramento In - Theis, Nesmith, Williams, Kabengale, Patterson, 2021 1st, 2021 2nd, 2022 2nd, 2023 2nd, Cash
Sacramento Out - Barnes, Robinson, Whiteside, James, Jeffries, 1.6 TPE, 3.0 TPE
Los Angeles In - Thompson, Teague
Los Angeles Out - Kabengale, Williams, Patterson
Boston cuts Javonte Green or finds a different team to take him
Boston Roster post trade
Starters - Walker, Brown, Tatum, Barnes, Whiteside
Rotation - Pritchard, Smart, Robinson, Williams, Ojeleye, Williams
Deep Bench - Langford, Edwards, James
Suits - Waters, Fall, Jeffries
Boston could use another big guy after these trades (and still has the full TPE to do it and about 7 million under the apron, plus still has some young players and all but the 21 1st available), but this would be much more about the long term rather than this season, and I think it would really force RW to play and see what he can turn into. Is he a potential starting center, I don't think anyone knows. He would get more minutes open to him with this trade.
So I'd do this trade to get Barnes who I think would be a very nice fit in the longer term around Tatum and Brown. Barnes would also open the possibility of a Smart trade and I'd certainly look at moving Walker at some point (this summer for sure) and see what I could come up with. The team needs pieces that will actually contribute to a title winner when Tatum and Brown are truly ready to lead the team where it wants to go.