Hi EandZ
Welcome to the Apple Community.
You might want to check the storage on your iPhone to see how many GB it would take to back it up.
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
_________________________________________________________________-
What an iCloud back-up includes:
What does iCloud back up? - Apple Support
________________________________________________________________
About storage on your device and in iCloud
What's the difference between device storage and iCloud storage? – Apple Support
____________________________________________________________________
Seems iCloud is mainly for syncing your data across various devices.
If you delete synced items from iCloud they delete on your iPhone, and vice versa.
There are other ways to back-up your iPhone data if you wish:
Backup methods for iPhone, iPad, and iPod touch - Apple Support
Eg:
Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support
All the best :-)