Make sure you get a Google Takeout, but realize that much of the image metadata is deleted (and sometimes outright changed) from the originals.

My takeout failed with an write until I requested an archive that was only photos, and only a year-subset of my library (which required me to make a manual album).

I also tried the open source "timeliner" project, (which does incremental backups very conveniently), but due to Google's API, all GPS tags are stripped from what the images that it can pull down.

If SyncThing doesn't work for you, you can also try Resilio Sync.

If you need software to manage all your photos and videos, you might want to try PhotoStructure! (I'm the author, details in my profile).