Is it possible to load single directories into iCloud? Triggered from the Mac Finder? I’m currently thinking about buying paid iCloud storage, because I’ve got several directories on my Mac which I’d like to have backed up that way.

My first look at iCloud was disillusioning, however. 

Obviously it isn't possible to upload whole directories via the web interface. Only single files. An alternative would be making compressed directories. But that option isn't great.

Is there a software application for desktop with which one can upload single directories?

I’d like to trigger the upload via the right-click context menu or something similar, and have the upload take place in the background. Ideally such an app would also “watch” the selected folder and keep it synchronized with iCloud.

Is this possible in the current state of iCloud Drive, and does a solution exist?
iCloud Drive on your Mac will upload the contents of your iCloud folder, including all subdirectories, to iCloud.

It is not possible to cherry-pick folders to exclude within your iCloud folder, or folders to include outside of it. Everything inside the iCloud folder gets synced with iCloud every time it changes, and that’s all.

Starting with macOS 10.12 Sierra, you may also specify the Desktop and Documents folders to be synced to iCloud. But those are the only exceptions. I believe the way this works is the original folders are replaced with symbolic links, and the actual folders moved into your iCloud Drive (I don’t use this feature, so I haven’t verified this myself).