Getting out of the tax is easy and will be done at the trade deadline. They will release Banton before the end of training camp, and then trade one of Walsh, Harper Jr, Scheierman, or Garza at the deadline (whichever is furthest from making the rotation. At least one of them will be out of the rotation). And then they will be under the tax, with enough salary space to add one player from the buyout market and another as the 15th man heading into the playoffs.
I mean, sure, they could make a larger move, but it will not be to avoid the tax. The tax is easy to avoid this year.
They still need to sign Cenac, but if they do that and cut/release Banton, I have them at around $1,200,000 or so over the tax if they start the season based on that. So yes, they could then trade someone with a contract in the $2.4M to $2.9M range and take nothing back. That would leave them at 13 roster spots. Then they play the 10-day contract or end of season contract game like they did last season. Keeping 13 as long as they can, and only having partial contract values count against the tax. Not sure I call that easy, but at least workable so long as they can find someone to absorb a contract at the trade deadline.
At this point, I may prefer to trade off Conley over Walsh, Scheierman, Harper, or Garza, but there is plenty of time to sort that out. Maybe Conley will surprise us.
I have them at $400k over the tax threshold once Banton is released. Cenac was signed over the weekend, so he is on the books.
Literally they could just release Jordan Walsh, who is not guaranteed, before the full guarantee date in January and replace him with Dillon Mitchell to get under the tax. That is unlikely, but my point is that it is super easy to get under the tax. They do not actually need to make a trade to do it. So while they are unlikely to take on extra salary in a trade this summer, any salary they move off will not be solely for getting out of the tax, because they have an automatic path if they want it.
EDIT: Just discovered a $1 million typo in my spreadsheet. Still, it is really easy to move a player at the deadline who makes the minimum. It only cost cash to get rid of Tillman, and the Celtics received cash for Minott, while Boucher was a future second-rounder for a recent second-rounder on a 2-way in John Tonje. Minimum salary contracts are very, very movable at the deadline, and the Celtics have many.