icloud syncing

Hi! I've had my new phone (iphone xs) for over a year now but we still use my old iphone for our kids to watch videos on at home via wifi. We recently tried to get all of the photos off of my old phone and on to icloud so the old phone would work a little better and was a little faster for the kids to use. For some weird reason, my old phone, new phone and icloud are all synced together. Once we uploaded the old phone's photos to icloud, they went on to my new phone as well. So now I have 27,000+ photos (old & new phone's photos) on my new phone and it is running SO SLOW.

When I try to delete pictures off of my new phone to free up space, it deletes the pictures off of icloud too. What. Is. Going. On.

HELP!