How do I back up my photos library to icloud without using iCloud Photos on my devices?

iCloud Photos doesn't put photos on your devices at a lower resolution if you don't turn on the Optimize Storage option. That is an option on each device; i.e., you could turn it on in one device if it was low on storage but leave it off on other devices. More information in Set up and use iCloud Photos - Apple Support

Also, iCloud Photos has some characteristics of a backup and has the advantage of being offsite, but be aware that if you delete a photo, accidentally or otherwise, it will be deleted from iCloud and from all of your devices and you only have approximately 30 days to recover it. I would recommend having a full backup of your Mac to an external USB drive.

TigerMoth1961 wrote:

That's a good idea, thank you. It's too bad that workarounds like this are necessary.

It has its positive side too. It forces me to prune my library and weed out mediocre shots. It is great to have a small photo library with just the shots that I want to show off, and to have exactly these photos in iCloud and on all devices. If our house should get damaged, I could recover the photos from iCloud. Another advantage is (coming next fall with Big Sur), that the captions we are adding to our photos will sync with iCloud Photos to the iPhone or iPad, and if you are taking new photos on the iPhone, you will be able to add a caption right away, and the caption will sync back to your Mac. I have waited a long time for this to be possible - taking a picture with my iPhone and being able to annotate it, while I still remember why I took it. (https://www.apple.com/macos/big-sur-preview/features/#photos)

I am only keeping my favourite Photos in iCloud Photos, simply by creating a Photos Library with just the photos I want to see on all my devices. My main libraries with all my photos are on an external volume. And the smaller Photos Library with only the favourites and the photos I am currently working on is my System Photos lIbrary and syncing with iCloud Photos to all my devices. The iCloud Photos library is small enough so I do not need the "Optimise Storage" feature at all.

I use both iCloud Photos and Google Photos. They both have positives and negatives. Google Photos will not automatically copy the album structure you have created in Photos on your Mac (or at least I haven't found a way that it will). Also note that iCloud always stores full resolution photos on iCloud while Google photos has options that can result in lower resolution photos on their system.