> To my knowledge we have to either charge these people with terrorism (definitely news), or wait until they’re abroad and do a drone strike (not viable with an imminent attack in the US).
Your knowledge is incorrect.
Insofar as a person is an enemy combatant in law, they can be militarily detained without charges regardless of where they are caught and what their citizenship is (and, if they are charged and tried, it can be by military commission), see, Ex Part Quirin, 317 US 1 (1942), concerning exactly this question with regard to German saboteurs in WWII, two of which were US citizens and all of whom were captured on US soil.
Also, insofar as they are enemy combatants, they may be engaged by the military with force anywhere in the world, regardless of citizenship and location. It would be silly if the military couldn't fight active enemies when they were in US territory but had to wait for them to leave, regardless of citizenship (for one thing, that would have made fighting the Civil War difficult for the Union.)