Syncing, or synchronizing, means keeping things in step. iCloud Photos keeps the pictures on all the connected Photos Libraries in step-- they have exactly the same pictures. If they aren't the same, it's not syncing.
But on a Mac you can have more than one Library! Many of us do. The Library called the "System Library" is the one one that gets synced. So, you could keep your old pictures in an OldPhotosLibrary, and create a new NewPhotosLibrary that syncs with your phone.
Many of us have archive Libraries for pictures that we've finished editing and aren't changing, And we have a Favorites Library, designated as the System Library, that syncs to the phone and iPad. We copy our favorite pictures that we like to see and share from the archive to the Favorites Library. And new pictures taken by the phone's camera will be in there too, because they are in sync. Some of the best new pictures stay in the Favorites Library, but all of them get copied to the appropriate archive library.
Maybe something like that is what you're looking for....
