I feel your pain. I didn't read how your files are organized on your card. Mine are by artist, subfolders for albums, and then songs, along with artwork (a pdf or sometimes folder.jpg). All three of my DAPs seem happy with that arrangement.
Your situation might be simply a defect. Does your R6 even see the card? Sorry for not reading all your previous posts on this issue.
i use the same way for the file arrangement like you. artist then subfolder with the album. at the beginning, it works without problem, but just until recently, it can't view by folder.
Have you tried a different app to play music such as power amp or neutron to see if that helps?
Tried with Neutron free player, it able to show by folder option but not hiby