Missing historical photos
I had all my historical photos nicely setup in iPhotos some years ago and when Photos was released I migrated across. I have from the start had everything synced with iCloud.
Some time ago when I was looking for an older photo I noticed that I don't have any photos pre 2014 and even 2014 is very sparse. Looks to me that all that exists is what was taken on on iPhone or saved on an iPhone and everything that pre-dates and was imported is missing. However the albums that held earlier photos (from the iPhotos import) are still visible.
I recently found the last backup I have from iPhoto which is 2015 but when I try to open this in Photos to import it gives me an error saying this library has already been imported.
What is the best way to import these photos and ensure they are not lost again? Ideally I would like to keep the album, face, location data that I had them organised by.
iMac (27-inch, Late 2013), macOS High Sierra (10.13.5), Macbook 11.13.5 & iPhone X