Hi PanMushra
welcome to the Apple (user-to-user) Community.
Re: "Downgrading my iCloud storage" : "I want to also cancel it"
How to: Downgrade or cancel your iCloud+ plan - Apple Support
says:
"If you downgrade or cancel your iCloud+ plan:
the change takes effect after your current subscription billing period ends*".
(*Partial refunds are available where required by law. If you recently changed your iCloud+ plan, contact Apple within 14 days for a refund. Learn how to request a refund.)
So it seems that after that period ends is when the storage amount would change.
Eg: you may want to back-up anything in iCloud you'd like to keep to a computer.
(Anything deleted from iCloud also deletes from anywhere that is in sync.)
and / or
You might want to turn off syncing anything you want to keep, before it deletes.
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Re: card : "I'd like to be able to remove it for my payment options"
If there are no unpaid charges, you will likely be able to remove your card when the current subscription period ends.
Eg: How to: Remove a payment method from your Apple ID - Apple Support
All the best :-)