New or Duplicate Photos Album...

The plan sounds good, but you will need a different disk for the archive of your Photos Library.

A Photos Library must not be on a NAS (or any other drive accessed over the local network).

The reason is, that Photos does not support the network access to a Photos Library stored on a network volume. The library needs to be on a locally mounted volume with a wird connection. Otherwise the library might get corrupted by inconsistent database entries due to race conditions.

To move a Photos Library to an external volume, prepare it as described here: Move your Photos library to save space on your Mac - Apple Support

And note Apple's warning:

"Make sure that your external storage device, such as a USB drive or Thunderbolt drive, is formatted for Mac: either APFS format or Mac OS Extended (Journaled) format.

You can't store your library on a storage device used for Time Machine backups. And to avoid possible data loss, don't store your library on a removable storage device like an SD card or USB flash drive, or on a device shared over your network or the internet, including over a cloud-based storage service."

As to your original question -

When you want to split the library, you will get the best results by saving a copy of your library on the external drive. Then delete the photos you want to remove from your library on the internal drive, once you have tested, that they can be read and worked with. By copying the library you will save all albums, projects, all metadata, faces, dates, locations, and the original image files will remain combined with the edited versions as one photo.

When you export the photos and reimport to another library, will lose the structure of the library, the projects, and some metadata.

If you want to keep the archive library small, you can also delete the photos that you have on your internal drive from the copy, but I prefer to archive the complete library, so I am having an additional backup of all photos.

Duplicating the library will work, as long as your library is not syncing with iCloud Photos. If you are using iCloud Photos and optimize storage, the copy of the library will be incomplete and the original image files will be missing for the optimized versions. In that case you have to export and import the the photos.

You may want to get a pair of USB/Thunderbolt drives, while you are about it. Your archive library cannot be the only copy, so you will need a plan, how to make backups of the external drive as well.

As your computer seems to be an iMac Pro, sitting on a desk, it will not be that bad to have the external drives connected at all times. It is more limiting, when we are using portable macs. It is tedious to have external drives connected, when carrying a MacBook Air or MacBook Pro around.