If you do use iCloud Photo Library, you can set to "Optimize iPad Storage", which will reduce the resolution and size of the files kept on the device, saving some space, and keep the full resolution photos in iCloud. However deleting photo's/videos from the device also deletes them from iCloud.
Settings > Photos & Camera > iCloud Photo Library > ON, Optimize Storage > ON
Note that to use iCloud Photo Library with the size collection you have, you'll have to purchase additional iCloud space beyond the 5GB free
You can keep all your iTunes music in iCloud, deleting them from the phone. You then can stream them if you have WiFi or cellular data but won't be accessible when you don't have internet access.
Settings > Music > iCloud Music Library > ON