Unfortunately, the Optimize iPad Storage feature, like the corresponding features on iPhone and Macs, is fully automatic and only seems to take effect when it notices that storage is already in short supply or when downloading a new photo from the cloud. There is no way to tell it "I need you to make extra room now." Also, even a fully optimized library needs some storage on the device; estimates range from 10-20% of the size of the library in the cloud, depending on a number of factors such as volume of videos versus photos.
How large is your iCloud Photos Library? You can check this in the iCloud storage portion of Settings. (The exact path depends on what version of iPadOS you are running now.)