Non system Photos library on the iCloud?

If you put a Photos Library on iCloud Drive and access it from a different Mac the syncing across devices may corrupt the library. I tested this with a few small test libraries, and none of them survived the syncing, after I opened them again while on iCloud Drive.

I would not risk that with important photos.

Guess it must be the same problem with other cloud services such as Dropbox etc.

All libraries I tested in Dropbox did not survive either.

If you want to have a Photos Library in iCloud, that is not your System Photo Library, you could upload the library to iCloud, then switch Photos to the library you want to use as your System Photo Library and disable iCloud Photo Library.

Now you can access the photos in iCloud using the web interface at https://www.icloud.com/#photos

The Photos.app on the webpage will let you upload and download photos, but to use the Photos.app on your mac for the photos in iCloud you will have to make that library the System Photo Library.

The Apple support documents do not say anywhere if the network access to Photos Libraries is supported or not. Fr iPhoto Libraries and Aperture Libraries are clear recommendations to use only locally mounted volumes; see for example this link: Use locally mounted Mac OS X Extended volumes for your Aperture library

Also, it is strongly recommended that the Aperture library be stored on a locally mounted hard drive. Storing the Aperture library on a network share can also lead to poor performance, data corruption, or data loss.

Since Apple is silent about Photos Libraries on a network drive, I looked into the Photos Libraries. The database files inside are SQLLite3 databases, just like in the iPhoto or Aperture libraries, and these libraries are not safe for network access. The Photos Library is a single user library just like the Apple's earlier photo libraries. I cannot recommend to put the library on a network volume.

Stay local is the name of the game. Pity!

Yes, or upgrade the storage (and pay) for a large iCloud Photo Library.