You might not be doing anything wrong...
As for Photo’s in iCloud, you need to understand that this service is intended to provide synchronisation of your locally stored photo’s with other device (such as your iPhone) - and usefully provides access to your photo’s should something untoward happen to your device(s).
A key point to note is that Photos in iCloud is not an external archive - independent of your device. Photo’s are deleted from iCloud (and any associated devices signed-in to iCloud with the same AppleID) as soon as they are deleted from a device.
You might choose to share with us details of your iPad model and its storage - along with how your storage is used by category. You can find your iPad model info in Settings > About - and your storage use at Settings > General > iPad Storage.
With a better understanding of the problem, we may be able to offer some constructive advice.