@ST . Thanks for updating your case. I don't see how I could help further as I don't have access (and never did) to a SharePoint site :(. All I know is what I read here and there
That said, the 7th pic. (pics. don't look the same but the process is the same) in SharePoint Folder clearly shows that when you use method Get Data > From File > From Sharepoint Folder, function SharePoint.Files is used:
and the above picture is consistent with the "From Folder" in Miguel's article Connecting to Files in SharePoint & OneDrive with Power BI / Power Query
Later in that article he says: You can use a function by the name of SharePoint.Contents which will give you a completely new experience which looks more like a way to navigate through your folders in the event that you have way too many files
IMHO this is exactly what you need so I would recomment you follow Miguel's article and use the SharePoint.Contents function
Hope this makes sense & helps