I would not trade Brown to the Mavs as I don't think there is enough value there but if you were ever going to consider that basic framework, you'd have to expand the trade
Brunson, Porzingis, Bullock, Burke, 22 1st, 24 1st (top 4)
for
Brown, Smart, Schroder, Fernando, Dozier
So Boston gets out of the tax, picks up some future 1st's and revamps the entire team
PG - Brunson, Pritchard, Burke
SG - Richardson, Langford
SF - Tatum, Bullock, Nesmith
PF - Porzingis, G. Williams
C - R. Williams, Horford, Freedom, Bol
8-man playoff rotation - Brunson, Pritchard, Richardson, Tatum, Bullock, Zinger, Rob, Horford
1 open roster spot though the team is just under the tax.
Again I wouldn't make that trade, but the value is at least better as it adds some 1st's, gets the team under the tax, and adds more long term financial flexibility (by lowering Smart's contract in dollars and length to Bullock). You'd have to feel ok with Zinger and Rob playing as the 2 bigs, otherwise you would never consider the trade, and while I like going from Smart to Bullock, I'm not sure the C's brass actually feels the way many fans do about Smart.