Backing up photos to external hard drive

I have a LOT of pictures on my iMac. Photography is a hobby so I have well over 60,000 pictures taking up well over 400GB.

All my files including Photos are regularly backed up to my Time Capsule. No problem there.

But the Time Capsule and my iMac are in my house. If the house were to burn down, they'd all be lost!

So, from time to time, I back up to an external hard drive that I keep in my shed!

The first time I did it, I dragged the Photos app from my iMac to the new external hard drive. After a few hours, all the photos had copied on the external hard drive.

Since then, I've accumulated more photos in the Photos app on my iMac.

So, how do I drag the Photos app over to the external hard drive such that only the new pictures are added?

Really important to me is not to lose the file structure of folders and albums that I use in Photos - that's how I very quickly find pictures I need.

Any solutions? Apple Support could only suggest using the Moments feature within Photos to export all pictures taken since the previous back-up into a new folder on the external hard drive. OK, that certainly backs-up the pictures but means that (a) the pictures are no longer all together, and (b) the folder structure is compromised.

Any help greatly appreciated.

G.