Completely by chance, I have stumbled upon the necessary option, on one of the screens of the Samsung Music app, this option offering one the chance to use "Tree" structure. After clicking this "Tree" option the app immediately diverted to my SD card and its folder hierarchy, allowing me to navigate my folder hierarchy however I choose. So now I can choose a category of music and tell Samsung music to shuffle the tracks in all the subfolders in that category, OR I can navigate to a particular subfolder in that category and only play the tracks in that subfolder.
The Tree option ought to appear in Samsung Music apps Settings section! It ought not to be buried away on just one screen deep inside the app, where it was pure chance that I came across it.