Use the Google Photos export, then write back metadata before importing to Synology Photos:
1. Export your photos from Google Photos using Google Takeout.
2. If you selected Storage saver mode or edited photo info in Google Photos, use Google Photos Takeout Helper to write the time and GPS information back to the photos.
3. Before running it, if you exported multiple .zip files, combine the contents into one folder.
4. Temporarily disable your firewall and antivirus software.
5. On Windows:
- Download the latest takeout-helper.exe from the project releases.
- Open the .exe and follow the instructions.
- Create a folder to save the results after running the .exe file.
6. Import the resulting photos into Synology Photos.
Note: this method does not apply to videos.
Sources used: [1], [2]