Hello hallacas,
Thank you for using Apple Support Communities. It's our understanding that you'd like to cancel your Apple One subscription and keep your iCloud storage subscription in order to protect your photos. We're happy to help.
When you cancel a subscription the changes will take effect once your current subscription billing ends. This will keep you from losing content while you change to your new storage plan. Once you've canceled the Apple One subscription we recommend that you immediately set up the iCloud storage plan of your choice to make sure your content won't be impacted once the billing ends. Use the steps in the following resource to cancel subscriptions and purchase iCloud storage:
How to cancel a subscription from Apple
Upgrade to iCloud+
Have a great day!