I’m trying to get my head around the L/D being independent of mass. Does lift scale with airspeed at the same rate as drag? Or is L/D only considering lift-induced drag (whatever the term is) and not total drag including parasitic drag?
Neither lift nor drag has anything to do with mass. They are entirely determined by the surface of the object, and are not affected at all by the interior properties, including density.
This is not the explanation you are looking for, but "aha the heavier object takes the same path but drops faster" was what made me okay with L/D not depending on weight.
Something a bit misleading done generally is aircraft don’t have one L/D, they have many, depending on angle of attack. When you see one number, it’s usually the best one.
I thought the point was that aerodynamics change from one domain to the next as shockwaves cause flow separation or eddies on or behind surfaces.