I initially started this as 2-team trade around JB for Bane. I then expanded it to see about getting under the 2nd apron and eventually the luxury tax. Not sure about the picks that should be involved.
Celtics in - Bane, Derick Jones Jr, Jay Huff, 2nd rounders (x3)
Celtics out - JB, Jrue, Hauser
Notes - Bane is good return for JB considering his better contract and he'd be a very good fit offensively. Huff is a solid backup C on a very cheap contract. Jones is decent filler. The main return is the nearly 49M cap reduction to get 6.6M below the tax line. 12 players on contract and the 6.6M would cover the 2 draft picks. Could adjust the trade to send Jones to the Nets and keep Konchar to get to 10.4M below the tax line which might allow Horford to re-sign. Could even send both Konchar and Jones to the Nets to get to 16.6M but that would certainly require another 1st.
Grizzlies in - JB
Grizzlies out - Bane, Huff, John Konchar, protected 1st
Notes - The Grizzlies need to make a big move to advance in the playoffs. Personally I'd trade for JB and then move Morant.
Nets in - Konchar, Bogdan Bogdanovic, protected 1st (Memphis), 2nd rounder
Nets out - none
Notes - Bogdanovic's 2nd year is a team option so the Nets aren't eating that much salary. They'd still have a ton of space to make more deals.
Clippers in - Jrue
Clippers out - Jones, Bogdanovic, 2nd rounder (x2)
Notes - The Clippers are stuck being all in and have limited assets to improve. Jrue would be a good addition to Leonard, Harden, Powell and Zubac. Went with 2 2nd rounders but maybe could get a 1st pick swap or even a highly protected 1st.
Mavericks in - Hauser
Mavericks out - 2nd rounder (x2)
Notes - I tacked Hauser onto this trade but he can be moved separately to a number of teams. Picked Mavs but I'm guessing the highest bidder will give up a couple 2nds.
Celtics roster would be: Tatum, White, Bane, Porzingis, Pritchard, Huff, Jones, Scheierman, Tillman, Queta, Davison and Walsh with 3 open spots.
I think this is a reasonable trade with some flexibility. Thought it would be harder but now I expect there are quite a few other reasonable trades that would result in big cap reductions while leaving us with a good core to build around. And I didn't even try to do anything with Porzingis.