Paying for iCloud storage.

No. You are renting the space, and to maintain your 50GB you have to continue the subscription monthly. How much data you put into it is up to you.

Think of it like renting a garage. You don't get another garage at the end of the month.

If you fill up the 50Gb allotment, you would need to upgrade to the 200Gb plan or the 2Tb plan. Or remove a bunch of stuff to free up space in the 50Gb allotment.