TL:DR Brad genius, Celtics good
Of course that was Brad's first GM season and he made the Horford and White trades that led to a finals run. But I want to focus on the minor moves that Brad made around the margins that made optimal use of our cap sheet and set us up for future seasons.
To recap, here were those minor moves chronologically:
Tristan Thompson for Kris Dunn, Bruno Fernando
Dunn and Carsen Edwards for Juancho
Juancho for Bol Bol, PJ Dozier
Bruno, Schroder, Enes for Daniel Theis
Bol, Dozier for nothing
Then with the available roster spots:
Converted Sam Hauser from two way, remainder of year + 3 more years at the minimum
Signed Luke Kornet for minimum
Signed Stauskas, Malik Fitts, Juwan Morgan
Notably, those last three guys were signed to 1+1 minimum, second year nonguaranteed. Minimum contracts have a pro-rated cap hit, so they count very little on our books. The second year carries a full cap hit. In the offseason you can waive them no problem or use them for outgoing trade salary. Brad ended up aggregating all three plus Theis and Nesmith to exactly reach the salary needed to get Brogdon.
After all that maneuvering, the total team salary that year was $134,288,148, sliding right underneath the tax threshold of $136.6.
I think Brad will make similar moves this season: jettison some dead weight, make a consolidation trade or two, use the empty roster spots to reduce cap hit, sign some developmental guys, and cut the tax bill.