Photos: 'Import All New Photos' fails when merging libraries

I'm not sure what went wrong. Have you tried it with selected pictures to see if it works any differently?

Are both Libraries located on your internal drive? If a Photos Library is on an external drive, then to avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Using Photos Import brings in the pictures and their metadata, but it does not keep any organization of albums and folders-- very disturbing for those of us who put lots of effort into organizing our Libraries. Most of us use the trusted 3rd party app PowerPhotos ($40) to merge Libraries, since that app maintains the album and folder structure. It's also great for copying specific albums or folders, or for groups of folders, even. PowerPhotos is so useful that I always keep it open while I'm using Photos.

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Yeah, I would say smaller batches. If you can avoid videos, put them off to last, that would help.

The problem with importing big chunks is that one incompatible file stops the whole thing. This may well be what happened to you. Video files are notorious for causing problems. The codecs for these things change without notice, and .mov doesn't always mean the same thing. Photos may have run into a file (video probably,) and just stopped dead to avoid loading troubling data. Unfortunately, it never tells you which file did it.

So, you go with smaller batches, avoid videos, and then if it stops you haven't lost so much time.