Removing photos from library

You can create multiple Photos libraries, or you could just store them on a couple different media to ensure they are backed up.

In either case, you would need to Export them from your Photos Library.

You could then create a new Photos Library and import them into it. Once imported, switch back to your other Library and delete them from that library.

Create additional photo libraries in Photos on Mac - Apple Support

If you have added a lot of metadata to those work files, i.e. titles, descriptions, keywords, etc., and want to keep the metadata your best bet is to use the paid version of PowerPhotos to copy the work Photos, the albums they are in and metadata to another empty library. That way you'll keep the organizational effort that you've put into them in your current library.