The Postmaster General is not an impeachable position, so impeachment is not an option in theory or in reality. As the USPS is currently constituted, the Postmaster General is more of the CEO of the company, though the USPS is not precisely a corporation in that sense.
The Board of Governors of the USPS does have the power to remove him, but seeing as they are the ones who put him in that position and the fact that most of them were Trump appointees, this appears unlikely for the moment.
There are several vacancies on the board and a couple more on the way. Enough, that if all the declared Dems plus filled vacancies all voted to remove DeJoy, he’d be out on his ass. More than a few Dems have been calling exactly for this to occur. The timing is less than ideal, however, as the process to fill sufficient seats would take the better part of this year. Those board seats require Senate confirmation, so the Dems winning the Senate was critical as well. Also, the Dems currently on the board voted to install DeJoy, so this isn’t exactly a foolproof plan.