Thank you both for your kind help @mlauber71 and @Add94 -
I finally got it working and the error I have encountered was based on me not understanding what String to Path node can do.
I was struggling with the fact that somehow I could only create LOCAL or RELATIVE TO paths but no CONNECTED paths. However, the latter is required by the List Files / Folders node if you want to read from an AWS S3 bucket dynamically.
But then it was simply a matter of adding an additional File System Connection Port to it and connecting it with the incoming connection from my Amazon S3 Connector node.
Now I only needed to loop through the different subfolders intelligently to get the files imported.
But both your comments somehow pushed me in the right direction, thanks again for that!