Problem managing apple one and icloud+ subscriptions

Hi. My father had bought a 200gb iCloud+ subscription, which was shared by our family. We decided to upgrade to Apple One. Now, we have a total of 400gb iCloud storage, which is not needed. We decided to cancel the 200gb subscription we had bought separately (not included in apple one), but we're not able to do so. When I click on 'downgrade options', the least I'm able to select is the 50gb plan. When I click on 'none' (the one in which only 5gb of free storage is provided), the 'done' button on the top right corner greys out. I've tried deleting all data, from iCloud, suspending family sharing, but nothing seems to work. Any help would be highly appreciated.

Edit: I've tagged the question under iPhone 12 mini, iOS 18, which is my personal phone. My parents both have an iPhone 15 (one 128gb and one 256gb). Mentioning just in case this information is required.