Plenty of space on icloud, but iphone too full with photos, what do I do?

You can connect your iPhone to your home computer via cable and your home computer will see your iPhone like a digital camera. You can then upload all the photos to your computer photo application.

Here's a link for that --> Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

iCloud storage is not the same as the physical storage on your iPhone. iCloud is for backups. If you need space on your iPhone, you'll have to delete content you don't need, such as photos, apps, music, email, messages, etc. Since you store your photos in the cloud, if you delete them from your phone, they will also delete from the cloud. To keep all your photos, you should consider uploading them to your computer directly or a cloud service, like Google Photos, Amazon Photos, etc., then delete them from your phone.

Do you have the Optimize iPhone Storage turned on in your iPhone? See the "Save space on your device" section of Set up and use iCloud Photos - Apple Support

You're welcome