The more I think about this question, the more I think the answer is:
They won't, other than hopefully getting healthier.
Really feels like a team like the Celts that has dedicated a huge portion of its cap to players the team drafted should get some kind of a break under the luxury tax, if not the salary cap overall. It sucks that the Celts are going to be so limited in what they're able to do because they drafted well and will be paying huge money to two young stars. Some of that is due to the cap remaining static as a result of COVID, but still.
Home-grown players should count less against the cap, at least for luxury tax purposes. That's my CBA "hot take."