Is it possible to pay for a full year of iClound rather than paying for it monthly?

Another option is to Add money to your Apple Account balance, which explains, "Use a payment method, such as a credit or debit card, to add funds to your Apple Account balance. Then use your Apple Account balance to buy apps, games, music, iCloud+, and more."

Not directly.

You can redeem a year's worth of gift cards onto your account, and it will use that balance automatically to pay for any subscriptions and purchases before it charges any other payment method. This includes iCloud, but also any other purchases or subscriptions you may have.