delete photo but keep in icloud

You can’t. That’s not how iCloud Photos works. All that iCloud Photos does is keep all of your photos in sync across all of your devices. If you have a computer, you can download the photos to the computer and them delete them from the phone.

Archive or make copies of the information you store in iCloud - Apple Support

If you don’t have a computer, you will be forced to delete other content to make room on your phone if you want to keep all of the photos.

You may also be able to save some storage on the phone by optimizing phone storage in the settings

Set up and use iCloud Photos - Apple Support

Well, you can, but you'd have to start uploading media manually to iCloud. Before you disable iCloud syncing on your iPad, archive all your media to an offline storage location, just in case to ensure you don't lose anything. Looks like you're only at 256GB right now, so that should fit fine temporarily on just about any modern Mac or Windows computer, assuming you have access to one.

The instructions here show you how to download the full quality "unmodified original" versions of everything manually from icloud.com:

https://support.apple.com/en-us/HT204264

After you have a backup copy in a safe place, turn off iCloud Photos on your iPad (reverse of the instructions in that article). The photos and videos should remain in both places (iPad and iCloud Photos), but if they don't for some reason, you have a backup. Then go back to iCloud Photos at icloud.com in a web browser from your iPad, and upload whatever's missing. Doing manual uploads will allow you to store more than what your device can hold. The downside is this becomes a manual process instead of auto-syncing, and you'll have to monitor your device's storage drive, cleaning it out periodically to keep it from filling up.

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Just thought of another option you can try first...

Under Settings->Photos on your iPad, you can select "Optimize Storage" (might say "Optimize iPad Storage"). What this setting does is allow you to keep syncing turned on, but when your iPad storage starts to run low, it will convert your local media to smaller resolution files that take up less space. The full resolution versions will stay in iCloud Photos. With this setting enabled, you might be able to buy yourself some time. and use up to 1 GB or more of space in the cloud before you are truly out of storage. Just keep in mind that if you ever need the full-quality version, you'll need to download the original quality from iCloud Photos.