Hello Ros,
Thanks for checking out the help article provided. Apple Photos and iCloud Photos utilize your entire library as a whole, but you have a couple options with Photos and libraries though. Before you start, be sure to back up your library.
One such option will be to export those older photos to the external drive. This will keep them in the Photo Library, but they will only be accessible with the external drive connected to your Mac. This article talks about this: Change where your files are stored in Photos on Mac - Apple Support
"If you prefer, you can store photos and videos outside the Photos library (for example, in a folder on your Mac or on an external drive) and still view them in Photos. Files stored outside your library are called referenced files."
For the original idea with moving libraries, but with your desire to only do this with half your library, your option is to create a separate Photos library. Create additional photo libraries in Photos on Mac - Apple Support
Either option will help you make space on your internal drive, but you will need to decide how to split the libraries up, export the files to the external, use referenced files or make the separate libraries.
If you really only wish to retain the older photo and don't plan on accessing that library very often, I recommend separate libraries. To start, first move your current library to the external drive, to regain the storage space on your computer. This keeps all your files intact and centrally located on the external drive. Do not complete step 6 though, as this will be your second library not for use as the System Library: Move your Photos library to save space on your Mac - Apple Support
Then, on your computer you can create a new library, and designate it as your system library. Create additional photo libraries in Photos on Mac - Apple Support
Please be aware, this will remove any and all photos from your computer and iCloud, and they will only be saved on the external drive. Then, use the article above for working with additional libraries, and export your most important new files from the external drive, to your new Photos System Library. When this is completed, only the newer photos will be saved on your computer and will then sync with iCloud, and be accessible in your other apps. When everything is completed, you will need to switch to your main library, and remember when you wish to move photos to the external drive or work with that second library, to connect your external hard drive, and switch the library using the process above.
To ensure you export the photos properly, and not just drag the thumbnails, please check this help article: Export photos, videos, and slideshows from Photos on Mac - Apple Support
I can't stress this enough, before any major changes to your photos and your libraries, I recommend you ensure you back up your library. Thanks and have a wonderful day.
