Thanks for the long description and reflection.
I like to 'spa' my feet as well. It started from over-use, over-use injuries, and other injuries. Even just separating the toes or doing "toe yoga" and "build" the "song" as well as structures/alignments from the ground up is interesting and helpful. Which makes me think that "fangsong" when there are pre-existing injuries or structural conditions is a bit harder, especially in the mechnical/structural/muscle aspects. How do you "sink" into your "kua" if there are some various injuries? For a while, I just tried to "follow the qi" and sink the qi as I need to build up my qi imho, and because it's somewhat "easier" to have the outside follow the inside. If I have the qi follow my yi, my limb (for example) already feels "song". But that doesn't necessarily address issues from some kind of injury or weakness, at least not at my level.