iPhone storage

More iCloud storage is usually not a solution for a shortage of storage on a phone. One way that it may help is via the option to optimize iCloud Photos storage. See the following from About storage on your device and in iCloud - Apple Support

Free up space on your device

You can make storage space available on your iPhone, iPad, or iPod touch when you use iCloud Photos and turn on Optimize Storage. Or you can remove content from your apps—like photos, music, and Podcasts—or delete the apps that you're not using.

Another option to free up storage space on your device would be to use Google Photos or Dropbox for photos and remove them from your device.

If you do buy more iCloud storage and decide to cancel, you can do that at any time and only be charged for the month (or months) that you were subscribed. Cancel by downgrading back to the free 5GB level: Downgrade your iCloud storage - Apple Support

iCloud storage could assist with making more room on your phone. It won't give the same amount of space but you can reduce the size of the Photos (using the optimize method), which basically saves a thumbnail on your phone and offloading the main photo onto the iCloud's server.

However, even that will have it's limits. Depending on your situation, this could be an improvement.

I cloud storage has no effect on phone storage. If you need more space on your phone, do not buy more iCloud storage. On your phone, delete things and move things to your computer.