You don't sign NDAs with the government, you sign a lifetime obligation [1] where the penalty is treason. I doubt you did or saw any such things.
1. https://media.defense.gov/2021/Oct/18/2002875198/-1/-1/0/NSA...
Actually security clearances do include an NDA. When I signed mine it contained an amusing clause, something to the effect of you will not share classified information until 70 years have passed or you die, whichever is _later_.
The existance and general operation of no such agency and echalon were common knowledge. I remember reading about them in Tom Clancy novels. Fantasy, but also widely understood reality. One doesnt need to have a clearance to count satalite dishes at like pine gap and realize what is happening.
If you are not a direct government employee you maybe sign an NDA.
I don't feel any compelling need to convince you.
During my career I signed dozens of NDAs. They were all either umbrella or caveat specific. All of them cited Title 18 referencing punishments (including death) for violations of the NDA, and all of them were related to either Title 10 or Title 50 activities.
Without being too specific, what I observed was the use of NSA assets to surveil grow operations within the US. It was explained to me that it began with Ronald Reagan's War On Drugs.
I've seen much worse since then while supporting Waived / Unacknowledged programs. Present classification requirements dictate that those be reviewed for declassification after 40 years, but they will never see the light of day because all documentation is destroyed at the end of the program and not archived anywhere.