The "Optimize storage" that léonie mentioned is a great way to go to keep all you pictures on your phone without taking up so much space. You can choose that on the Mac, too, to save storage space, there.
Another way to go may be to keep two Photos Libraries on your Mac. One, the un-synced one, would have all your older pictures, while the other library, synced to your phone through iCloud, would have more recent pictures. There are some complications with this, though.
The synced library must be the one designated as the "System Library," and it is the library that is used by other apps to access your pictures--like for screensaver images or uploading to the internet. Of course, in addition to recent images, you can add whatever pictures you like to the System Library, and they will appear on your phone, as well.
Another complication is that the "Old Picture Library" would be your only copy of those pictures. To be safe, you would need a copy of this library on an external hard drive.
If you're interested in doing that, see this article:
Create additional photo libraries in Photos on Mac - Apple Support (IE)