How to Transfer an Entire Photos Library to a New Apple ID

Switching to a new Apple ID presents challenges when transferring your entire Photos library. While Apple offers some tools, they can be unintuitive. This guide explains the most efficient approach for moving your Photos library without tedious manual backups.
Avoiding the Manual Backup Problem
I initially attempted copying photos in batches, discovering that Apple restricts bulk transfers to approximately 1,000 photos at a time. After completing a manual backup, I discovered a more efficient hidden option that streamlines the entire process to just a few straightforward steps.
Steps to Transfer Your Photos Library
Step 1: Share the Library with the New Apple ID
Open the Photos app on your current Apple ID and establish a Shared Library. Invite your new Apple ID to participate in this shared library.
Step 2: Accept the Invitation with the New Apple ID
Sign in using your new Apple ID, open the invitation, and accept it. Allow the Photos app to synchronize the shared library content.
Step 3: Leave the Shared Library on the New Apple ID
After synchronization finishes, exit the Shared Library from your new Apple ID. When prompted, select the option to "copy all shared photos to your library."
This approach leverages Apple's Shared Library functionality to transfer photos when departing the shared library—significantly faster and more reliable than manual export or backup methods.