This is probably fairly obvious but it looks like i have missed it.
Is there an option in adobe arcobat DC to automatically apply an encryption/security policy to a document after it being signed?
This would be to avoid having to manually apply protection each time a document is signed.
You can use a script, but it has to be backed by a folder-level trusted function, and it won't work in Reader, of course.
You must apply security BEFORE using a digital signature because setting security destroys a digital signature. But with a digital signature (as opposed to a stamp) security is pretty much irrelevant; the point of digital signatures us to detect change, not prevent it. Especially as much software just ignores the security.