Oh, I should hasten to add that I have nothing against using Google Photos as a "backup backup." The default settings for Google Photos generally reduces the resolution of the photos that most phones take pretty significantly, although they're just fine for snapshots and sharing with friends. You can set it to keep original resolution but that puts a strict size limit on what Google Photos will store for you.
I always copy my photos (or cut and paste if I don't want them to remain on the device) to my computer as the backup, and have a routine full system image backup routine for the computer itself.
I don't care what the device is, including disk drives - nothing can be relied upon as the sole long term storage location for any electronic media. If it's important you have a minimum of two copies. If it's critical, you have three, with one of the backups kept in a location where the source itself and primary backup is not kept.