Ensure you have enough disk space and bandwidth available for downloading the Shared Album to your Mac.
For migrating a shared album with 4800 photos and videos, I needed approximately 20 GB free disk space.
Create a dedicated account for the migration on your Mac.
This is an optional step. It is useful if you want to back up only the photos and videos from the iCloud Shared Album to Google, and not the other photos from your System Photo Library.
In the System Preferences, under Systems & Groups, unlock to make changes and click the + button to add a user account. Log in with this account. When macOS prompts you to log in to your iCloud account, use an iCloud account that has access to the shared album.
Open Photos, select Shared Albums, and wait for the Shared Album to be synced.
In the meantime, deactivate iCloud Photos in the Photos Preferences, so that your photos and videos won't be uploaded and stored to iCloud another time.
You can monitor the download activity in the Network tab of the Activity Monitor.
Copy the photos from the Shared Album on your Mac (only the photos, videos will be tackled in the next step.)
See Archive or make copies of the information you store in iCloud:
a. Open Photos and select Shared Albums.
b. Double-click the shared album.
c. In the top right corner, next to Showing Only, choose Show only Photos.
d. Select all photos (Command-A).
e. Control click (or right-click) a photo and choose Import.
Copy the videos from the Shared Album to your Photos library.
a. Create a folder on your Mac, e.g. on the Desktop.
b. Open Photos and select Shared Albums.
c. Double-click the shared album.
d. In the top right corner, next to Showing Only, choose Show only Videos.
e. Select all videos (Command-A).
f. Drag-and-drop the videos to a folder on your Mac.
g. With all videos still selected in Photos, open the File menu and, under Export, click Export Unmodified Original for all Videos. Ensure that Export IPTC as XMP is selected in the dialog, and confirm the Export in the same folder.
This creates XMP files containing the date for your videos. The Photos app currently needs this workaround with external XMP files so that the date information does not get lost when you import videos.
h. Import the videos back from the folder to Photos: In the folder, Sort by type or Group by type in the View menu, and select all the video files: Only the video files, not the XMP files. Drag-and-drop the videos to Photos.
i. Delete the folder you used to transfer the videos from the Shared Album to Photos.
Use Google Backup and Sync to automatically backup photos from your Mac.
Select Upload newly added photos and videos to Google Photos in the preferences.
In the Network Settings, it is possible to limit the Upload Rate.
This procedure won't copy comments, though.