I am planning to use NextCloud for the sync, as I like its YEAR/MONTH/FILE structure, and configuring it as an external library in Immich. I could also use NextCloud Memories, but Immich UI is nicer, and its face/object recognition model is better.
but you say it can also use external libraries? so it does not copy those?
Yes, it only renames files automatically sync'd from your phone. My understanding is it creates a separate metadata file for each external file, but doesn't touch the actual files in an external library.
So if you have existing backups just add them as external libraries.
as far as free, it's absolutely free to spin up. it does require registration (like any other service), but Cloudinary has a generous free tier. it was made pretty clear on the website and repository it relied on that service
just for transparency, i work at Cloudinary, but I stand by the above whether it was Cloudinary or another service