No difference from a security/privacy perspective. The 8a has a longer support time since it released later than the 8 and 8 Pro though.

The differences between the versions mostly come down to camera, display, screen and build quality and battery size. Pro has more RAM.

The Secure Camera app built into GrapheneOS uses CameraX and has almost all features and the same camera quality. It's the only open source app we recommend. Others who don't make that choice would use the Pixel Camera for certain features.