Can I delete iPhone data after iCloud backup?

Be very careful! I'm not exactly sure what you are wanting to do. You can not think of iCloud as independent storage! And iCloud is not a real backup!

iCloud Backup will replace your last backup in one day, using what is now on the phone. That means, if you erased data on your phone, then you will lose everything!

iCloud Photos mirrors your phone. If you delete pictures on your phone, they will be deleted at iCloud!

See this:

Understanding iCloud Photos - Apple Community

and this

Understanding iCloud Drive - Apple Community

Let us know what you really want to do, and we may be able to help.

yes you can, just make sure you have anything save on your iPhone, our make download in your computer, just for precaution