Copying a library between the devices is problematic, because of the file ownership and system versions. You can only copy the library to a mac running the same system version or a newer version, but not to a mac with an older version. And create the copy on an external drive with the "Ignore ownership on this volume" flag enabled. Prepare the external drive as described here: Move your Photos library to save space on your Mac - Apple Support
The big advantage of iCloud Photos is that it allows us to keep the Photos Library identical and in sync across all devices, including the metadata, the structure (albums, smart album, folders), the adjustments. iCloud is taking care of the file ownership and the differences between system versions. I can start editing my photos on the iPad and continue on any of my macs, either on a Mac with Mojave or Ventura. I do not have to apply the adjustments manually on all devices, or to recreate the albums and folders, add favorite hearts again, add titles and keywords and create inconsistencies. My Photo Stream synced only the unadjusted originals.
For the slideshow projects or books I would suggest to create a separate Photos Library for each of your projects, at least for the new projects. Keeping a project in a separate library makes it easier to archive and will protect it from accidental deletion of photos that are used in the project. When we are weeding out duplicates or similar redundant photos, it is easy to accidentally delete a photo that is needed for a slideshow project. If you want to profit from the syncing, use iCloud Photos: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community
If you just want to transfer new photos between devices, it should suffice to download new photos using a USB connection or Wi-Fi.
When I create an elaborate slideshow, I am using Keynote to create the presentation. The Keynote presentations are syncing with iCloud to all my devices and I can see them on my Macs, my iPhone, my iPad.
