Hello! You can back up your photos by:
1. In photos, press command + a to select all photos
2. Create a new folder and drag your photos in there. This may take some time and your photos app may become unresponsive for a few seconds if you have a lot of pictures.
3. Upload the photos to Google Drive!
I hope that this solved your issue, and if you were to have any further questions - do not hesitate to ask!
Regards,
Alexander @ TechBuddy Sweden