Can I delete my photos and videos from galary once after done the back in iCloud

Warning:

Read this article carefully:

What does iCloud back up? - Apple Support

Photos may not be part of Automatic iCloud Backups”

Rather, photos are often (and usually by default) continuously “sync’d” via iCloud Photos.

If iCloud “sync” is turned “on” … when you delete them from your phone … it will also delete it from iCloud.