What are safe files to delete to free up space?
Technically, you can delete every thing in the User Library and the system will continue to work just fine. A brand new user account has nothing in its Library, after all.
The user Library contains iCloud Drive files, iOS backups, user-specific app data like Mail and Messages, as well as settings and preferences.
But realistically, the small amounts of space you can save here and there (and the constant manual management) is not worth the effort: the biggest occupier of space is your apps and your files. If you're filling up a small internal storage drive, then you need to think about using external drives, network or cloud-based storage.