If I had to guess its going to be Giannis to Miami for Herro, Ware, Jaquez, Jakucionis, 29 1st, 31 1st and a few swaps as well. As for the Celtics I think they'll look around and see what sort of value White, Hauser ect have before the draft because moving off some money could open up the option of using the 27-million-dollar TPE.
If you forced me to guess for the rest of the offseason I'd guess they more or less stand pat. They pick someone at 27, they trade #40 for a future second(s), they use the MLE on someone and stay just under the tax or over by a small enough margin that they could get out if need be. Maybe they trade Hauser for draft capital since they do have a lot of wings. It won't be flashy but I don't think the team is that far away.
That said there is a lot of variance though in offseason outcomes. The above I think is most likely, but a Giannis trade would not surprise me. Neither would a White trade to get younger. A Jaylen non-Giannis trade would surprise me; I don't think they are trading him just for assets. You only trade him if you bring back a clearly better player.