How can I move my iPad photos to iCloud?

Consider that iCloud is principally a data synchronisation service. If you have enabled iCloud Photos, your photos are already stored in iCloud - however, if deleting photos from the iPad, the deletion will be synchronised with iCloud. If you have not enabled iCloud Photos, your locally stored photos will be included in your iCloud backup (if enabled). The danger here is that if photos are deleted from the iCloud backup next time that the backup is refreshed.

Photos and other files can be manually moved to iCloud storage independently of your iCloud Photo Library and/or iCloud backup, but this is perhaps not the most appropriate method to address your difficulty of releasing storage to perform an system software update. If saving files and photos independently to iCloud is of interest to you, due to limitations in the allowed length of a post here, I'll follow-up this aspect in a second reply.

Addressing your immediate problem...

It is generally recommended that you maintain at least 4-6GB of free space at all times - as your iPad needs some temporary space to complete some tasks - such as performing system software updates.

A system software update requires sufficient space to be downloaded, unpacked and verified prior to installation - major version updates (such as updating from iPadOS 17.x to iPadOS 18) requiring considerably more space than an incremental point-update. After the update is successfully completed, temporary files are automatically deleted - returning free space.

If your internal storage is full, or is insufficient for the task that you need to perform, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for a software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

External storage options can often help with managing limited internal storage. Depending upon your iPad model, the iPad will either have a Lightning or USB-C port - and may therefore require either an Adapter or USB hub to connect the USB Storage device. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB3 Camera Adapter:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/xc/product/MK0W2ZM/A

Part 2...

While you can enable iCloud Photos to (a) synchronise your Library between devices and (b) access you library from any computer using a web browser - using iCloud Photos does not provide the ability to keep your photos/videos just in iCloud. You must understand that deleting items from your Photo Library will cause the corresponding copy to also be deleted from iCloud Photos. As such, iCloud Photos is not suitable for use as a independent repository for your photos and videos.

While iCloud Photos is not, of itself, suitable for maintaining a device-independent location to store your photos/videos, you can still use your available iCloud storage to save your media. To do so, you will need to export your photos/videos from your Photo Library to a folder that you create in your iCloud file storage...

Using the native Files App, create a folder in iCloud - within which you will initially save a copy of your selected photos and videos.

Next, open your Photos App - and from your Library, tap to select the items that you wish to copy to iCloud file storage. When you have selected the items that you wish to copy, tap the Share button (a square with an upward-pointing arrow) at bottom-left of the Library window.

Tapping the Share button will expose an action menu; from the menu select Save to Files - and navigate to the iCloud folder that you have created - then tap Save. Your selected photos/videos will be copied to the folder that you have selected.

Don't try to copy all your photos/videos in one go - as this will be very slow and may fail due to insufficient system resources. Instead, copy across small batches.

When you have finished, check that all your photos are visible using the Files App. You should note that photos and videos that are not stored within the Photo Library cannot be accessed or viewed using the Photos App.

When you are happy that you have copied your photos/videos to the intended folder, only then is it safe to delete the original image files from your Photo Library.

See this support page about that matter:--> Back up and sync your photos with iCloud (extracted from:--> iPad User Guide )

Regards

Giulio

As per my experience, the best instructions in order to:

• create a safe copy of your files from iCloud;
• make room on iCloud storage (if needed);

are listed in this support article:--> Archive or make copies of the information you store in iCloud

When done, if you accidentally delete something synced between your iPad and your iCloud, that "something" is always recoverable from the copy you created.

Regards

Giulio