Photos on iCloud

I have a number of Apple devices, including a newer IPad Pro (128 GB) and an older iPad (13.0 GB). I also pay for 200 GB ICloud storage. I have almost 12,000 pictures. All of my devices are synced, so al, the pictures are stored in lo-res on all of them. My older iPad’s storage is nearly full and consequently is very slow in operation. I would like to take all the pictures off the older iPad to free up some storage, then use it for surfing and reading my ebooks. How can I clear out the memory, while still retaining the pictures on my newer iPad, iPhone and Mac. I went on iTunes and de-authorized the old iPad, but when I tried to remove pictures last night, it deleted them from both the new iPad and the phone. Can anyone suggest a way to accomplish my task?