I think Zapier probably can do what I need, but it is a little over my head.
I think what I do is have Zapier trigger Dropbox to append a new line (containing a new folder name) each time a new folder is created in one of my client folders. Zapier’s Dropbox actions seem to allow the appending of data to a file natively. Then things get a little fuzzy: I somehow pickup the content of this file and populate my choose from List action. I guess this is where URL hooks come in? Or can I fill some kind of Shortcut variable woth the content of a file saved on Dropbox via a Shortcut action?