The self pardon and the pre-emptive pardon have not been tested at the Supreme Court.
The Department of Justice could argue that a self pardon is not legitimate. An they could also argue that pre-emptive pardons, especially ones that have not been vetted by the DOJ Pardon Attorney, have no bearing on future prosecutions.
While the current SCOTUS would most likely side with Trump on the issue, they are very wary of the precedent it might set, especially since the DOJ has argued (and the courts concurred) that a sitting President cannot be tried. Let the former President stand trial, and be pardoned later if warranted.
Trump may be charged with treason; setting up a system where a President can commit Treason and then not ever be prosecuted for it would be against the thought process of the Founders.
It would make Trump apoplectic if Biden announced on January 20 that he was appointing Hillary Clinton as Special Prosecutor.