Can I delete my photos after I have stored them on my iCloud Drive?

No.

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.

That depends on how you actually stored them there.

If you used the iCloud Photo Library option, then that syncs all changes, so anything you delete from the iPhone will be deleted from iCloud.

You can turn off the iCloud Photos option in Settings ➜ Photos to prevent the deletions on your iPhone from Syncing to iCloud and removing the photos from there too.

If you used the Export feature in the Photos app, and used the Save to Files option, then you can delete the Photos from the Photos app without affecting the iCloud Drive content. However, those photos would still be taking up local storage through the Files app, so not sure anything would be gained by doing that.

If you did something else, then you will need to explain exactly what you did before a definitive answer can be given.