I mentioned it in a different comment, but take a look at Syncthing. It does mesh-style backup to synchronize a folder between multiple machines. That provides robustness against hard drive or PC failures, and it's easy to add an offsite node for extra confidence.
You can use Takeout to bulk-download photos: https://support.google.com/accounts/answer/9666875?hl=en
I don't know if you can automate syncing Google Photos to a local disk. Wouldn't be surprised if there was.
(edit: Wow. Lots of people wanting to help! I wasn't expecting two sibling responses.)