I'm trying to figure this out myself. I've gone from Google Photos -> iCloud -> testing out Unraid + PhotoPrism + NextCloud. That is, point PhotoPrism's import folder to the NextCloud photos folder. Both hosted on Unraid. And then on my iPhone, have the photos automatically uploaded. Only issue so far is I don't see an auto-detect and import option in PhotoPrism so I have to manually click 'import'.

Several of my beta users have recommended PhotoSync. I've used Resilio Sync on my iOS test device successfully as well.

Most NAS providers (like Synology and QNAP) also have native sync apps.

It's good to have options so if one gives you attitude, you can switch it out.

I wrote up more details here: https://photostructure.com/faq/how-do-i-safely-store-files/#...

Not sure about the upload part, but for getting photos out of iCloud, this works:

https://github.com/icloud-photos-downloader/icloud_photos_do...