What is FlowLogix MirrorImmich? It's a macOS Photos -> Immich sync tool.

FlowLogix MirrorImmich extracts title, description, keywords / tags, GPS location and album information created in Apple macOS Photos and creates a directory that includes symlinks to photos and their extracted metadata in sidecar .xmp files.

This directory can then be rsync’d to Immich external library.

Features

• Creates a directory structure to be used as an external library in Immich.
• Exports titles, descriptions, GPS location, and keywords / tags.
  • Note: Appends titles and descriptions, since Immich only has one description field.
• Generates .xmp sidecar files.
• Exports edited photos, if they are edited.
• Creates albums in Immich based on Apple Photos albums.
• Marks favorites in Immich based on Apple Photos favorites.

Edit: I was asked for some links:

Official Web Site

MirrorImmich on GitHub