Not an answer to OP; I use iCloud, but I still want my own, external backup of that and have found some good solutions.

I previously used icloudpd which I run from docker and that synced all files to a NAS from where I took backups. It works great and I highly recommended.

However, I recently turned on Advanced Data Protection on iCloud, which disables the API icloudpd relied upon, so now I’m using osxphotos instead. It works just as good.

You’ll have to turn on downloading all originals to your Mac for this to work. Personally I had to buy an external disk and move the photo library on my Mac mini server to it to have enough storage.

And don’t care about privacy / believes that Plex will not be hacked one more time.