You may want to look at the trusted 3rd party app PowerPhotos ($30) that makes importing Libraries much easier. It preserves the album structure, as well. I bet it would work. The free trial may support this, but I don't remember back that far.
I asked those last questions for these reasons: iCloud Photos is great for synchronizing my iPad and iPhone with my Mac, but Optimize Storage comes with some complications. For instance, Optimize may keep smaller images locally to save space, so the full Library can't be backed up. I don't use Optimize on my Mac, but I do on my phone and iPad where space is more limited. But I want to keep one Library "optimize" free.
On an iMac, it's very reasonable to keep the Photos Library on an external drive to save space on the internal drive, if you need to. But the drive must be properly formatted. If you intend to use an external drive, see this:
Move your Photos library to save space on your Mac - Apple Support
For a laptop, however, using an external drive for an iCloud synced Photos Library has its own issues.