Need at least the camera, if not that plus a microphone and speakers, not the lock or bell.
Libre Software (Obviously) + End-to-End Encryption
Small and easy to hide, so the camera isn’t stolen, attacked or bypassed.
Ops. Missed the actual details. Sorry. How small you want it to be? Any dimentions?
No specific measurements.
https://www.wyze.com/products/wyze-cam-pan
Something like this which has a privacy mode where it rotates and hides could be good.
Vandal proof camera is easy to find, but with privacy mode? I don’t think such product exist yet.