What's the correct way to back up all Photos from the Photos app?
The Photos app in macOS Sierra is able to do a comparison between what is in the cloud, and what is stored in the local library on your Mac. (Likely through some means of signature/hash function).

To provide some insight with actual numbers - My Photos library is approximately 53GB, and only required about ~1.5GB worth of bandwidth to validate/sync the library after migrating the library to a different Mac with a fresh install of macOS Sierra (1.5GB includes any photos that it also needed to sync since the last my library was active). 

With that being said, you can be confident that your bandwidth limitation, or capacity, should not have to handle the full file size. However, be mindful that if there are other cloud services (iCloud Drive) they too can utilize bandwidth.