Putting active duty military members on Title 10 orders, such those of the 82nd Airborne Division which reporting indicates Trump also wanted to use in 2020, would also be an obvious violation of Posse Comitatus if those troops were used to arrest, surveil, or otherwise control protestors. Active duty troops may perform in a supporting function for domestic law enforcement, such as by providing engineering, logistics, or communications to law enforcement, which is what US troops do when they're on the border (or when they are supporting counter-smuggling operations).