To add to the discussion, the Porzingis TPE expires on July 7, 2026. That give the team 1 week after the end of the season to sign or trade for someone using this mechanism. That is a legitimate avenue, but one that is potentially difficult, given the short time frame.
Trading Simons is not technically the only way to have his salary slot at the team's disposal but it would buy more time and flexibility. For example trade Simons using the Porzingis TPE to bring a player back, then you would get a new TPE for Simons salary (or whatever the net ended up being), and the timeline to use that would be extended. In that way, you could potentially have a player or players back for Simons whose contracts extend beyond the end of 2025/26 plus have a TPE to use in the 2026 off season or the 2027 trade deadline to use on another player. Lots more options.
It is not likely that BOS would be able to do all of the above, convert Simons expiring into 1 or more contracts that extend beyond the end of 2025-26, plus add another player with a new TPE from trading Simons, but the flexibility would be there. As it stands now with the Porzingis TPE, you either trade for a player between now and the deadline, or sign someone for 2026-27 between Jul 1 and Jul 7.