I have not looked at the APIs closely:
Requested an API-key via my university email, as a researcher interested in materials about “Hessians”.
Got a key, but only 10.000 requests per month
https://catalog.archives.gov/api/v2/api-docs/
How much info did you need to give them to get a key? I’m employed in an archive (not in the US) so maybe I can also request one from a ‘professional interest’ angle.