I think I understand that you have iCloud enabled on your Mac Photos System Library. Soon, your Mac Library will have on its internal drive all the pictures that are in the iCloud Library. You have another set of pictures on you iPhone, but iCloud is not turned on for Photos there. Is that right?
If so, then when you turn on iCloud on Photos on your iPhone, all the pictures on the phone will be added to the pictures already in iCloud Photos in a process that will likely take a few days. You will then have 70,000 pictures in the iCloud Photos Library, and there might be duplicates. You will also have 70,000 pictures in you System Library on your Mac, and 70,000 pictures on your phone. iCloud synchronizes the Libraries so that they all, phone, mac, iCloud.com, have the same pictures.
Since the pictures will be stored in iCloud, you will not need to have full resolution images stored on your phone. Once you turn on iCloud on your phone, you can go to Settings>Photos and turn on "Optimize iPhone Storage." This will keep pictures on the phone that look great on the small screen, but don't take up so much storage. The full-resolution images will always be available at iCloud.com.
If you want to delete your pictures on your iPhone, you will need to disconnect iPhone Photos from iCloud, or all those new images will be deleted at iCloud.com and on your Mac, also. iCloud is about synchronization-- whatever you do on one device happens to all of them.