he can terminate all the union contracts based on him saying so?

He sprinkles "national security" around when he wants to abscond with employees' paychecks.
If federal unions haven't been disbanded, they've got a shot at challenging him in court, so that he has to produce the evidence to prove
a) what constitutes 'national security' and 'mission' in the USDA (recall that Cheney opined that"food is a weapon" ), and
b) that USDA workers are an essential part of both definitions, although
c) the union was already formed during all the time the evidence existed, and thus, itself, could be called "essential" to any USDA national security mission.

The long game is Project 2025's goal -- to privatize government. He's being horse whispered by Vought who's being horse whispered by Charles Koch, who's owner of the largest private industry in the US.

that try to de-unionize govt so it can be "optimized and privatized."

He does this to poison the national food supply, to kill more domestic exports... what country in the world would willingly import food they can't trust as being "good"... let alone, nutritious. And to kill us in thousands of different ways, overt and covert. Think of all the fungus infested federal buildings, for example. It took our union to finally get the govt to shut down and send everyone home to work, so that they could remediate the mold levels in the building I used to work in about 10 years ago. This same building used to house the Buffalo Soldiers barracks, and before that, was the horse stables. There are so many ways to harm workers and civilians alike without unions watching our backs.

People will die from these decisions. They will sue for damages.