I am trying to create external backups of all my digital data in case something bad were to happen. Is it safe, or even beneficial, to compress a .photoslibrary file to store on an external device for later use, or would it be better to save the file as it is?
All the data is highly compressed and binary. I would just save it off as it is, Time Machine or any other file copy is sufficient and efficient in storage to back up your Photos library data within the library.
It is a package which is a fancy name for a structured folder on disk, so that’s why it’s easy to copy off using a variety of tools.