Hi,

There is, however you pay a bit more than when you fully pay the amount up front. https://proton.me/pricing here you can find all the subscriptions which are available

The discount in price is the only benefit of the two year plan from what I saw. They can offer this discount because they can get the lump sum of money in one go. This is the idea of the time value of money, a lump sum now is greater than an equal amount later.

It makes little sense to offer a month to month plan at the same price with a contract in place, because now there’s the overhead of the contract on top of everything else.

So your best option is likely to stick with the month to month price, unfortunately.