How many people do you think would need to be quiet about a flat earth conspiracy? You do realize it's more than a few astronauts?
NASA has about 20.000 employees plus 100.000 people working for private contractors, there are other space agencies in the world of course, European, Russian, Chinese, Japanese, Indian, ...
You would need to also silence many scientists, cartographers, airplane pilots, ships crews, at least parts of the military, engineers in fields using satellite information like television, communication and GPS ...
The NSA employs around 40.000 people and produced an Edward Snowden not too long ago ...
How save would the FE conspiracy secret be?
Not even mentioning that we knew about round earth long before NASA existed ...