I have ~30,000 photos in my library, which takes up ~90 GB disk space. My MacBook has limited hard drive size, so I'm using an external hard drive for my Photos library. The library is synced to iCloud.
This means I have two full copies of my photo library - the external drive and iCloud. But I still don't feel perfectly secure. I have heard of people who have lost large parts of their photo collection when absentmindedly deleting photos from one iOS device to free up space, only to realize that iCloud syncs the deletions to all other devices.
So I would like a third backup location, so that I can be reasonably sure I'll have a good backup of my entire photo library in case of disaster or screw-up. I would of course want everything to be as automated as possible, so that my photos are backed up every day (or on any modification) without me having to do anything.
These are the options I have researched:
Create two partitions on the external drive - one for the Photos library, one for a Time Machine backup of the same. This would allow me to revert modifications to the library. Cons: Having a Time Machine backup and a Photos library on the same drive is not recommended by Apple, so I would need yet another HDD.
Use a 3rd party online backup service to mirror the entire Photos library. This allows me to recover from any disaster happening to my iCloud and/or local library. Cons: The Photos library consists of not only image files, but also a SQLite database file, which in my case is about 450MB. Every time I modify my library in any way, the database file is modified, meaning that I need to upload (at least) 450MB to the backup service for every single thing I do. This seems like a huge waste of bandwidth, and the backup is hardly incremental.
Keep the actual Photos library on the internal HDD, while having the image files as "referenced" images, stored on the external HDD. This would save space on the internal drive. Cons: Would require me to organize the file structure myself. iCloud sync is also not possible for referenced images.
Are there any better options for achieving a good automated, incremental backup for macOS Photos?