Sorry for not responding as I haven't been online since then.
In the end I decided it is best to make each library the System Library in turn and upload the photos to iCloud. Eventually all the photos will be in the cloud and I can add them to my main library.
However I have now hit a new problem.
I have just gone through this entire process on a Mac Mini running Mojave. I had hundreds of GB of photos spread across 4 different library files from various years. My goal was simple, put them all in one library. Here are my thoughts on your situation and this thread so far.
1)
Photos Export option - For me, the export option from Photos app was by far the worst option. It
does not retain original quality, and it takes ten times longer than simply copying, because it is essentially creating brand new versions of all your photos as JPEGS (or whichever format you choose). This is processor intensive, time-consuming, destroys any high-quality RAW format files you may have, and I have also found is unreliable for retaining metadata. It's odd that metadata is only
sometimes preserved, but ultimately too unreliable for me.
NOTE: If you really want to use photos to manage data transfer, there is an option to
"export unmodified originals" in the Photos menu. This option is VERY fast, as it simply copies originals for you, as if copying files in finder. If you have done a lot of edits within the Photos app, may want to also export those edits. You can isolate only edited photos in the main Photos section, and Photos tab, and on the top-right side change "Showing: All Photos" to "Showing: Edited". Then you can select all of these, and export normally, preserving originals & edits.
2)
Power Photos - This was my preferred solution because it automates and preserves the MOST POSSIBLE metadata, albums, etc. I chose to copy originals and not edited versions (as you must choose one or the other). While, I have edited many photos, it's not enough for me to care to preserve ALL edits. If I ever want to use or print an old photo, I can always edit it to my liking if needed. That said, I did preserve MANY edits by setting my largest existing library as the destination library, with all other libraries being merged into the largest one. As I chose to copy originals only, all is not lost, as I can still open the smaller source libraries and use the method above to export specific edited files directly from Photos if I really want to.
Major caveats!
1 - Always perform operations with COPIES of libraries. Never your originals - because ometimes there are errors. I've had a couple instances in the past where libraries got stuck in an infinite "repairing library" screen, and after days of waiting, I had to just force-quit Photos app, delete the Photoslibrary and start again from a backup. Always keep backups, especially before attempting merging, or other major management tasks.
2 - I have disabled ALL iCloud options for photos, so all of my process considers local files only. I have no idea how having iCloud options enabled would mess with this process or Power Photos.
I'm not sure which new snag you have run into, but I hope it works out! Good luck!
~j
