I seriously doubt Johnson and the rest of the GOP leadership in the House or Senate has the balls to even threaten a shutdown, let alone actually take a stand and do it.
The House rank and file would have to defeat any CR without the SAVE act, and usually there are enough defections among the RINOs to pass anything the Dems really want.
If there ever was a time to shut it down, it would be now. People will understand a shutdown if slightly less crooked elections is the real goal.