The Celtics have a (potential) pick swap in 2028 with the Spurs (as a part of the White-trade), so they can still trade picks in '25, '27 and '29. Rotation for the Celtics would become:

Doncic/White/Tatum/Horford/Timelord (+ Brogdon, Hardaway, Hauser, G-Will(if resigning)/Bertañs, Griffin(?)) That's definitely a championship team right there.

For the Mavs a reunion between Irving and Brown is interesting to say the least. However getting an wing All Star in his prime in return is something very few teams can offer. Add to that the current DPOY in Smart and that's a combo which would prevent the Mavs from necessary tanking.

The Celtics take on the bad contracts of Hardaway and in particular Bertañs. Obviously a lot of teams can offer much better picks, it depends on which direction the Mavs would want to go. Perhaps a third party with heavy interest in Brown and/or Smart (but not having the pieces to make an offer for Doncic) could round off the deal.

I do not expect the Mavericks to trade Doncic. Even if he's really disgruntled. But if they would, in practice there are few teams who can make a compelling offer for a player of that caliber while being left with a team that's not completely dismantled.