200GB iCloud Drive is full, I do not wish to pay for 2 TB plan. How do I prevent some files from backing up to iCloud?

If you only want your photos and specific files to sync to iCloud, that can be accomplished by turning on only iCloud Photos and iCloud Drive. It sounds like you may also have turned on the Desktop and Documents option for iCloud Drive on one of your iMacs. The following has information about that and about how to turn that option off (read that part carefully if you decide to do it). See: Add your Desktop and Documents files to iCloud Drive - Apple Support

re: "I am being forced to upgrade the storage or not be able to backup my iPhone, iPad or other iMac computers."

Apple does not offer a cloud backup service for computers. iCloud Drive is a syncing service, and while it does offer the advantage of having an offsite copy, it is not a backup. Any change or deletion that you make to a file will be immediately synced to iCloud. Unintended deletions cannot be recovered if you do not notice them within 30 days. I use iCloud Drive but I also have backups to USB external disk drives.

re: "From what I read, it appears if I delete the desktop file from iCloud it will also delete it from the iMac as well."

See the following from Add your Desktop and Documents files to iCloud Drive - Apple Support

Turn off Desktop and Documents

When you turn off Desktop & Documents Folders, your files stay in iCloud Drive and a new Desktop and Documents folder is created on your Mac in the home folder. You can move files from iCloud Drive to your Mac as you need them, or select all of your files and drag them to the place you want to keep them.

1. From your Mac, choose Apple menu  > System Preferences. Click Apple ID, then click iCloud. On macOS Mojave or earlier, choose Apple menu  > System Preferences, then click iCloud.
2. Next to iCloud Drive, click Options.
3. Deselect Desktop & Documents Folders.
4. Click Done.

If you turn off iCloud Drive or sign out of iCloud, you have the option to keep a local copy of your files that are in iCloud Drive. Whether you decide to keep a local copy or not, a new Desktop and Documents folder is created in your home folder. If you choose to keep a local copy, your files in iCloud Drive are copied to a folder called iCloud Drive (Archive) in your home folder. Then you have the option to move any files that were in your iCloud Desktop and Documents, back to your new local Desktop and Documents.