Q: How do I make synced copies of photos into proper deletable copies?

The premise of iTunes photo sync is that the photos are managed from the computer; e.g., future syncs with no changes in the iTunes settings will add and/or delete photos from the phone to match any additions and/or deletions done on the computer. There are more details, including how to delete them from the phone, in Sync photos from your computer to your iOS device using iTunes - Apple Support

If iTunes Photo sync isn't what you want, there are other methods of copying photos from a computer to the phone. They include 3rd party apps such as https://photosync-app.com/ and cloud services such as iCloud Photo Library FAQ - Apple Support and 3rd party cloud services.