Nextcloud doesn’t like changes on disk in its own file structure, but you can mount “external storage” where Nextcloud is okay with changes and happily scans the location when you access it (a network share, or a local file path also works; SMB share will probably get you around the permissions problem though.)

Don’t know about immich as I haven’t used it, but you will probably have to decide on one of the two services to be “in charge” of the files, I think.