Question about upgrading to iCloud+

I was wondering if I make the upgrade and use it for a few months, but then decide to discontinue the monthly plan, what happens to the files and pictures that are backed up and my access to them?

You are already using the free 5GB plan. If you commence but then cancel your subscription to an iCloud+ plan you will revert to the 5GB plan.

Also, are the iCloud+ plans truly a month to month plan, or is it committing for a period of time like a year or 6 months?

It is a monthly subscription and can be cancelled or downgraded (to a lesser plan) at any time.

Say I decide to have the upgrade, and I get all my pictures backed up. then can I delete all the pictures from the iPhone so I can have more space on my phone? and are all the pictures I delete still stored on the iCloud?

iCloud Photos is not dependent on having an iCloud+ plan, other than you require sufficient space to house your photos. iCloud Photos is a syncing service not a backup. If you delete photos from one device it deletes those photos from ALL your devices - including iCloud. Best not. Your iCloud space is independent from your iPhone. See What’s the difference between device storage and iCloud storage?

I have a debit card, no credit card, can I still join iCloud+ and pay directly each month without a fixed payment plan on my card? Is that ok? (meaning I will need to authorize each month a new payment)

The payment to Apple for iCloud+ is an automatic payment using your preferred payment method chosen in your Apple ID. Credit or debit cards are both accepted.

Say I can make the monthly payment on a different date each month, is that ok?

No, it's a fixed day of the month and the 'anniversary' date of your first payment.

See Upgrade to iCloud+ for the details on the three paid plans and their cost. It's a small investment for your important data and services.