You need to protect against (1) the hard drive dying, (2) the whole house burning down, (3) bit-rot making some of your photos unreadable over time, (4) ransom-ware encrypting or deleting your photos.
Here's what I did:
I set up a small cheap PC 'backup server' with ZFS on Linux and two mirrored 4TB drives. Dropbox keeps my photos synced directly to this backup server and a simple script copies over any new photos to a folder. Another simple script makes ZFS snapshots of any new files or changes every 15 minutes. On ZFS this takes almost no space or time, only the changed disk blocks use any space. I set up a second identical backup server and put it in my office. If my office didn't allow that I could have put it at my brother's house. My server does a ZFS send periodically to keep the servers in sync. The server does a weekly 'zfs scrub' to check the file checksums.
The mirror makes sure a dead hard drive doesn't lose my files. The snapshots make sure if someone accidentally deletes files or a ransom-ware gets them I can get them back. The scrub protects against bit-rot. The zfs send to the second server protects against the house burning down.
Alternatively if you don't want to mess with Linux and ZFS, just use Crashplan's free option and have it store a second copy of your backups on a second computer at your office or friend's house.