He needs to do this as fast as possible. Label it as aimed to quite the unrest in the country over the election. That should make it hard for Biden to fire them later. He would need to appoint someone he trusts. Not sure who would be good for this.

Edit: I understand it is likely they will be fired. Republicans have always been bad at creating bad optics for their opponents. Doing this creates a win/win for conservatives. Win if they allow the investigation to go through and win if they kill it. There is no downside. You just have to be careful who you appoint to not make it easy to fire them. Maybe someone like Robert Barnes. I prefer Sidney Powell in this case but too easy to fire her.