My solution to that very problem is to just use a third party app to sync to my NAS. I can't use Time Machine since my Macbook Pro doesn't have my full library downloaded to it (when will Apple ever start with 512 GB SSDs in their laptops...).

I have iCloud Photos and pay for it, but I want to make sure I have a local backup too so I use NextCloud to sync to my NAS (which is running a NextCloud instance). You could also use the Plex app, or there's things like Resilio Sync that can auto-upload photos.

I'm not aware of an open source solution for auto-upload other than NextCloud (and possibly OwnCloud, but I don't use it), but I wouldn't be surprised if there was one somewhere or an app that did auto-backup to SMB/CIFS/WevDAV/whatever

Yeah, I guess I'll switch to Synology Moments, but it's still significantly more hassle than using the nice Google Photos app and having a rather hassle free local backup.

Time Machine. My Synology NAS backs up (including Time Machine data for all of our family MacOS devices stored on the NAS) to Backblaze B2.

“Lock down” is hyperbole. You can mount Apple filesystems (including the one storing your photo library) from a Linux workstation or server over the network.

I admit that if you’re looking for an open API for photo management, iCloud is not for you. But it works for most people.

How does a photo storing system where I can't share pictures with my Android owning grandmother "work for most people"? Are my family members who didn't give out money to Apple good enough to share my life experiences with?

https://www.chriswrites.com/share-icloud-photos-non-apple-de... (How to Share your iCloud Photos with non-Apple Devices)

I share my Apple Photo albums with android users without issue. I hope this was helpful despite your snark. I’m just trying to help.